UNITED STATES ENVIRONMENTAL PROTECTION AGENCY 

WASHINGTON D.C., 20460 

OFFICEOF • 

CHEMICAL SAFETY AND 

POLLUTION PREVENTION 

MEMORANDUM 

DATE: 

SUBJECT: 

FROM: 

THRU: 

TO: 

March 10, 2014 

Registration Review- Preliminary Problem Formulation for the Ecological Risk 

Assessm~nt and Drinking Water Exposure Assessment to be Conducted for 

Imazapyr and Imazapyr Isopropylamine (PC Code 128829 (isopropylamine salt) 

and 128821 (imazapyr acid); DP Barcode 41 7327) ~ a /I' A ~ /t ") 

James A. Hetrick, Ph.D., Se~ Adviso~ · / / ' vf 1 

Tanja Crk, Biologist / ~ ~~ L/'t:.- "Stl b 2/JIJj 

Environmental Risk Branch 3 · J 

Environmental Fate and Effects Division (7507P) 

Dana Spatz, Chie~ , / ;. 

Melissa Panger, Ph.D., Senior Scientist 3/ / 6 If 

Rosanna Louie-Juzwiak, RAPL (}) _ 

Environmental 

Environmental Risk 

Fate and 

Branch 

Effects 

3 

Division 

~ 

(7507P) 

Joel Wolf, Chemical Review Manager 

Kevin Costello, Chief 

Risk Management and Implementation B"ranch 2 

Pesticide Reevaluation Division (7508P) 

The Environmental Fate and Effects Division (EFED) has completed the preliminary problem 

formulation for the ecological risk to be conducted as part ·of the Registration Review of 

imazapyr and imazapyr isopropylamine. At this time, EFED does not expect to conduct a new 

drinking water assessment given that the Health Effects Division (HED) does not anticipate 

conducting a dietary assessment. This document is intended to provide an overview of what is 

currently known regarding the environmental fate and ecological effects associated with 

imazapyr and imazapyr isopropylamine and their degradation products, and outlines 

uncertainties regarding attributes of the parent compounds and their transformation products. It 

describes the preliminary ecological risk hypothesis and the processes that will be used during 

the completion ofihe ecological risk assessment in support of Registration Review. 
2 

1. INTRODUCTION 

The active ingredient imazapyr is a systemic imidazolinone herbicide that is used for control of 

most annual and perennial broadleaf weeds and grasses, woody species, and riparian and 

emergent aquatic weeds species. It is formulated both as an acid (imazapyr) and as an 

isopropylamine salt (imazapyr IPA). The mode of action of imazapyr is to inhibit 

acetohydroxyacid synthase (ALS) which interferes with plant cell growth and DNA synthesis. 

 

The chemical structure of imazapyr along with the chemical identifiers is provided in Table 1. 

Table 1. Imazapyr Chemical Identifiers 

Chemical Name Imazapyr Imazapyr IPA 

Chemical 

Structure 

Chemical 

Abstracts Service 

(CAS) Registry 

Number 

81510-03-0 81334-34-1 

Imazapyr is currently registered for use in terrestrial (railroads and industrial right-of-ways, 

fencerows, wildlife habitats and forests) and aquatic (ponds, lakes, reservoirs, marshes, bayous, 

canals, streams, rivers, and water drainage systems) non-crop sites and for corn fields. Aqueous 

imazapyr formulations may be mixed with surfactants or oils for application, as well as mixed 

with other herbicides and fertilizers. Imazapyr is also available in a water dispersible granular 

formulation and an emulsifiable concentrate. Typical terrestrial application methods consist of 

ground and aerial spray, with granular broadcast applications for forestry uses, while surface 

waters including bogs, marshes, potholes, wetlands, and intermittently flooded areas are treated 

directly with aqueous formulations. For more information on use and usage, please see the 

BEAD Chemical Profile document. 

2. PREVIOUS RISK ASSESSMENTS 

An ecological risk assessment in support of the re-registration eligibility decision (RED) for 

imazapyr (both acid and salt formulations; PC Codes: 128821 and 128829), hereafter referred to 

as imazapyr, was finalized by EFED on September 30, 2005 (DP Barcode: D313607) (Table 2). 

The screening level risk assessment indicated risk to non-target terrestrial plants (monocots and 

dicots) and aquatic vascular plants from imazapyr use, based on the highest application rate from 

a variety of use patterns. Seedling emergence and vegetative vigor for both monocots and dicots 

would be impacted by exposure to both the imazapyr acid and the isopropylamine salt. The 

assessment indicated minimal risk of acute effects to fish and aquatic invertebrates and minimal 

risks to aquatic non-vascular plants at maximum application rates. In addition, there were no 

chronic risks to fish and invertebrates, although there was an uncertainty for estuarine/marine 
3 

fish and invertebrates, since no toxicity data were available to observe the prolonged effects of 

imazapyr to these taxa. Likewise, acute and chronic risks to mammals and birds consuming food 

types containing imazapyr residues are not expected from the labeled uses of the herbicide. 

EFED currently does not quantify risks to terrestrial non-target insects; however, available data 

on honey bees indicate that the risk to terrestrial non-target insects was likely to be low. The 

assessment indicated the potential for indirect risk to all taxa from direct effects on plants (i.e., 

effects on habitat and/or primary productivity). 

An assessment of risk to the listed California Red-Legged Frog (CRLF, Rana aurora draytonii) 

was completed on July 20, 2007. That assessment indicated no expected direct effects on either 

the aquatic or terrestrial phase CRLF. There were also no indirect effects expected for the CRLF 

through direct effects to either its terrestrial or aquatic food sources. The assessment determined 

that the CRLF may be adversely affected through direct effects on habitat and/or primary 

productivity (i.e., ecosystem structure and function for both the aquatic plant community and 

riparian vegetation). Critical habitat may also be adversely modified based on direct effects to 

aquatic vascular plants and terrestrial plants. The risks exceeded the level of concern (LOC) for 

non-listed non-target terrestrial plants (monocots and dicots) for all imazapyr uses. The risks to 

non-listed non-target aquatic vascular plants exceeded the LOC for aquatic, rangeland and 

forestry uses (aerial application) as well as rights-of-way (assuming 50% pervious surfaces). No 

effects were expected for aquatic non-vascular plants. 

A Special Local Needs 24(c) assessment was conducted in May 24, 2010 (DP Barcode 

D374578). The request was to allow broadcast application of imazapyr (Arsenal PowerLine 

EPA Reg. No. 241-431) on grass pasture for control of cogongrass (Imperata cylindrica) in areas 

that may be grazed or cut for hay. The assessment indicated no expected risks for effects to 

aquatic and terrestrial animals from the proposed broadcast spray application of 0.75 lb a.e./A of 

imazapyr on pastures. Non-target terrestrial plants may be adversely affected from the proposed 

use. Off-site movement of imazapyr onto non-target terrestrial plants is associated with both 

runoff onto adjoining fields and spray drift deposition. 

Finally, an assessment was conducted in August 2, 2010 (DP Barcode D379032). BASF 

petitioned to add aquatic use sites to the Arsenal PowerlineTM herbicide (EPA Reg. No. 241-431) 

label to allow direct application of imazapyr to potable surface water, public waters, private 

waters, livestock watering ponds, recreational water areas at a maximum rate of 1.5 lb a.e/A per 

year. 

Table 2. Summary of Previous Risk Assessments Conducted by EFED on Imazapyr 

Use Reference 

(DP Barcode) Max. Application Rate 

Taxa for which risk was 

identified at the Agency’s 

concern levels 

Various RED, 2005 

(D313607) 

1.5 lbs a.e./A Terrestrial plants, aquatic 

vascular plants; fish, aquatic 

invertebrates, aquatic nonvascular plants 

Various CRLF, 2007 1.5 lbs a.e./A CRLF assessment: Terrestrial 

plants and aquatic vascular 

plants 
4 

Table 2. Summary of Previous Risk Assessments Conducted by EFED on Imazapyr 

Use Reference 

(DP Barcode) Max. Application Rate 

Taxa for which risk was 

identified at the Agency’s 

concern levels 

Grass pasture SLN 24(c), 2010 

(D374578) 

0.75 lb a.e./A Terrestrial plants and aquatic 

vascular plants 

Direct aquatic D379032 1.5 lbs a.e./A Terrestrial plants, aquatic 

vascular plants; fish, aquatic 

invertebrates, aquatic nonvascular plants 

Previous Drinking Water Assessments 

The drinking water assessment (DWA) for imazapyr was conducted on terrestrial (spot treatment 

on pasture) and aquatic uses (D275562, 2003). The DWA was based on an application rate of 

1.5 lb a.e./A, which represents the maximum application rate for aquatic use sites (Table 3). 

Table 3. Estimated Drinking Water Concentrations (μg/L) for Imazapyr

Use Pattern Models Peak Annual Average 

30-year Annual 

Average

Aquatic/Terrestrial@ 1.5 lb 

a.e./A (D275562, 2003) 

Tier I FIRST 137 81 NA

SCI-GROW 35.9 NA

NA=not applicable for a Tier 1 drinking waters assessment 

 

Clean Water Act Programs 

Imazapyr is not identified as a cause of impairment for any water bodies listed as impaired 

under section 303(d) of the Clean Water Act 

(http://water.epa.gov/lawsregs/lawsguidance/cwa/tmdl/index.cfm ). No Total Maximum Daily 

Load (TMDL) criteria have been developed for imazapyr. Aquatic life benchmarks have 

published for imazapyr 

(http://www.epa.gov/oppefed1/ecorisk_ders/aquatic_life_benchmark.htm) (Table 4). The 

aquatic life benchmarks represent ecotoxicity endpoints derived from risk assessments to support 

pesticide registrations. 

Table 4: OPP Aquatic Life Benchmarks for Imazapyr 

Compound 

Fish Invertebrate Non-Vascular 

Plants 

Vascular 

Acute Chronic Acute Chronic Plants 

μg/L 

Imazapyr >50000 43100 50000 97100 12000 24 
5 

3. ENVIRONMENTAL FATE AND TRANSPORT 

The environmental fate database is complete for assessing the environmental fate and transport 

of imazapyr and imazapyr isopropylamine (Appendix A). Imazapyr is an anionic, organic acid 

(pKa of about 3.8) that is non-volatile, degrades through photolysis in clear shallow waters, and 

is both persistent and mobile in soil. A summary of selected physical and chemical properties 

for imazapyr and imazapyr isopropylamine salt are presented in Table 5. Imazapyr is mainly 

present in anionic form at typical environmental pHs, and the behavior of the acid and salt forms 

are expected to be similar, therefore, the environmental fate of the imazapyr is evaluated in terms 

of acid equivalents. Most environmental fate data available for imazapyr are based on 

dissociation of the isopropylamine salt in water. Imazapyr was essentially stable to aerobic and 

anaerobic soil metabolism, and no major transformation products were identified during the 

course of laboratory studies (Table 6). Field study observations are consistent with imazapyr’s 

intrinsic ability to persist in soils and move via runoff in surface water and leach to groundwater. 

Imazapyr did not bioconcentrate in submitted laboratory studies. The relatively high solubility in 

water and low n-octanol to water partitioning ratio of imazapyr is also consistent with little 

likelihood of bioconcentration. 

Table 5. Physical and Chemical of Imazapyr and the Isopropylamine Salt of Imazapyr

Physical and Chemical Properties

Chemical name Imazapyr Imazapyr Isopropylamine Salt 

2-[4,5-Dihydro-4-methyl-4-(1-

methylethyl)-5-oxo-1H-imidazol-2-yl]-3-

pyridinecarboxylic acid 

2-Propanamine, 2-(4,5-dihydro-4-methyl-4-(1-

methylethyl)-5-oxo-1H-imidazol-2-yl]-3-

pyridinecarboxylate 

Empirical 

Formula 

C13H15N3O3 C13H15N3O3·C3H9N 

Molecular 

Weight 

293.16 g/mol 322.19 g/mol 

Aqueous 

Solubility at 25 

C 

11.1 g/L MRID 00145872 

pKa 3.8 MRID 00145872 

Vapor Pressure 

at 60 C 

<10-7 mm Hg MRID 00145872 

Henry’s Law 

Constant at 25 

C 

<7 x 10-17 atm x m3/mol Calculated 

Log Pow at pH 7 

and 20 C 

0.22 MRID 00145872 
6 

Table 6. Environmental Fate Data for Imazapyr and Imazapyr IPA 

Guideline # Data Requirement 

(material) 

MRID # Values 

835.2120 

Hydrolysis 

(Imazapyr) 00132359 t 1⁄2=Stable 

835.2240 

Photodegradation in Water 

(Imazapyr) 00131617 t 1⁄2-=2.3-3 days 

835.2210 

Photodegradation on Soil 

(Imazapyr) 40003713 t 1⁄2=149 days 

835.4100 

Aerobic Soil Metabolism 

(Imazapyr) 

(Imazapyr) 

41023201 

45119701 

t 1⁄2=5.9 years 

t 1⁄2=296 days 

835.4200 

Anaerobic Soil Metabolism 

(Imazapyr) 00131619 t 1⁄2=Stable 

835.4400 

Anaerobic Aquatic Metabolism 

(Imazapyr) 40003712 t 1⁄2=Stable 

835.4300 

Aerobic Aquatic Metabolism 

(Imazapyr) 

(Photoproducts: CL 9140, CL 

119060) 

41002301 

41891501 

45119702 

t 1⁄2Stable-Parent 

t 1⁄2=5.7 days-CL9140 (Upper 90% CB of 

Mean) 

t 1⁄2=8 days-CL119060(Upper 90% CB of 

Mean) 

835.1230 

835.1240 Leaching-Adsorption/Desorption 

(Imazapyr + photoproducts) 45119705 

Sand-Kf=0.091 (1/N=0.733) Kfoc=19.4 

(Parent) 

Silt loam-Kf=0.091 (1/N=0.887) 

Kfoc=19.4 (Parent) 

Sand-Kf=0.69 (1/N=0.811) Kfoc=149 

(CL9140) 

Silt loam-Kf=31.6 (1/N=0.955) Kfoc=4940 

(CL9140) 

Sand-Kf=0.551 (1/N=0.939) Kfoc=117 

(CL119060) 

Silt loam-Kf=5.77 (1/N=0.962) Kfoc=902 

(CL119060) 

835.6100 Terrestrial Field Dissipation 

(Imazapyr) 

(Arsenal 2AS) 

(Arsenal 2AS) 

45119706 

42192101 

42192102 

 t 1⁄2 =143 days – OR field study (soil) 

 t 1⁄2= 64 days – NC field study (soil) 

 t 1⁄2= 94 days-IA field study (soil) 

 t 1⁄2=126 days-NE field study (soil) 

835.6300 

Aquatic Field Dissipation 

(Imazapyr) 

(Arsenal 2AS) 

45119707 

41891501 

 t 1⁄2 = 3 days- FL field study (water and 

sediment) 

 t 1⁄2= 4 days- LA field study (water and 

sediment) 

835,6400 

Forestry Dissipation 

(Arsenal) 40003714 

 t 1⁄2 =24 days- AL field study (soil) 

 t 1⁄2 =26 days- AL field study (soil) 

850.1730 

Accumulation in Fish — 

No Data 

Nonguideline Accumulation- aquatic nontarget 

(Imazapyr) 

(Imazapyr) 

45119707 

45119709 

Fish/Crayfish- No detectable 

bioaccumulation 

Shrimp/Oysters- No detectable 

bioaccumulation BCF<1 

 
7 

Transformation Products 

Although imazapyr is stable to hydrolysis and microbial metabolism, photodegradation in water 

is an important route of dissipation. Major photodegradation products of imazapyr (>10% of 

applied radiation) are CL11960 and CL9140. The degradation product CL252974 was a minor 

transformation product (<6.9% of applied radiation) in hydrolysis and aerobic soil metabolism 

studies. The transformation products CL11960 and CL9140 are prone to degrade through 

oxidative mineralization to CO2. These transformation products are listed in APPENDIX A

with the maximum amounts of each observed in the various environmental fate studies. 

4. RECEPTORS 

The toxicity endpoints for imazapyr acid for use in the risk assessment are shown in Table 7. 

Available aquatic ecotoxicity study endpoints on imazapyr salt and acid are summarized in 

Table 8 and Table 9. The available terrestrial ecotoxicity study endpoints on the isopropylamine 

salt of imazapyr, and its acid form, are in Table 10 and Table 11. A summary of the ecotoxicity 

data is shown in APPENDIX C. The endpoints from the toxicity tests with the isopropylamine 

salt of imazapyr are expressed in acid equivalents (a.e.). 

Table 7. Toxicity Endpoints Used in Assessment of Risk in Acid Equivalents 

Organisms Toxicity 

Endpoint 

MRID 

Aquatic Organisms 

Acute Freshwater fish LC50 (mg ae/L) >100 00131629 thru 

-631 

Acute Freshwater invertebrate EC50 (mg ae/L) >100 00131632 

Acute Marine/estuarine fish LC50 (mg ae/L) >184 41315801 

Acute Marine/estuarine invertebrate (mollusk) EC50 (mg ae/L) >132 45119710 

Acute Marine/estuarine invertebrate (pink shrimp) LC50 (mg ae/L) >189 41315803 

Chronic Freshwater fish NOAEC (mg ae/L) 43.1 41315804 

Chronic Freshwater invertebrate NOAEC (mg ae/L) 97.1 41315805 

Aquatic nonvascular plants EC50/NOAEC (mg ae/L) 11.5/7.16 43889102 

Aquatic vascular plants EC50/NOAEC (mg ae/L) 0.018/0.011 43889102 

Terrestrial Organisms 

Acute Avian (oral dose-based) LD50 (mg ae/kg-bw) >2150 00131633 

00131634 

Acute Avian (dietary) LC50 (mg ae/kg-diet) >5000 00131635 

00131636 

Acute Mammal (oral dose-based) LC50 (mg ae/kg-bw) >5000 00132030 

Chronic Avian (dietary) NOAEC (mg ae/kg-diet) 1670 45119714 

Chronic Mammal (dietary) NOAEC (mg ae/kg-diet) 10000 41039505 

Chronic Mammal (dose-based) NOAEL (mg ae/kg-bw/day) 738 41039505 

Acute Terrestrial Invertebrate (contact) LD50 (g a.e./bee) >100 00131637 

Dicot Seedling Emergence (EC25)/(NOAEC/EC05) (lbs ae/A) 0.0024/0.00017 40811801 

Dicot Vegetative Vigor (EC25)/(NOAEC/EC05) (lbs ae/A) 0.0009/0.000064 

Monocot Seedling Emergence (EC25)/(NOAEC/EC05) (lbs ae/A) 0.0046/0.00099 

Monocot Vegetative Vigor (EC25)/(NOAEC/EC05) (lbs ae/A) 0.012/0.0039 
8 

Receptors 

Tables 8 through Table 11 provide a summary of the aquatic and terrestrial taxonomic groups, 

respectively, and most sensitive surrogate species tested to characterize the potential acute and 

chronic ecological effects of imazapyr. 

Effects to Aquatic Animals

Table 8 provides a summary of endpoints from aquatic toxicity studies for imazapyr acid. The 

available toxicity data for the salt are no more toxic than the acid and are summarized in 

Appendix C. 

Table 8. Summary of Endpoints from Aquatic Toxicity Studies for Imazapyr Acid

Taxonomic 

Group 

Study 

Type 

Species % 

ae

Endpoint Toxicity 

Category / 

Endpoint 

Affected

MRID No. 

Author/Year

Study 

Classification

Freshwater 

Fish 

Acute 

Bluegill 

sunfish 

(Lepomis 

macrochirus)

93 96-hour 

LC50: >100 mg/L 

Practically 

 non-toxic 

00131630 

ABC 

Laboratories, 

1983 

Acceptable 

Rainbow trout 

(Oncorhynchus 

mykiss)

93 96-hour 

LC50: >100 mg/L 

Practically 

non-toxic 

00131629 

ABC 

Laboratories, 

1983 

Acceptable 

Channel catfish 

(Ictalurus 

punctatus) 

93 96-hour 

LC50: >100 mg/L 

Practically 

non-toxic 

00131631 

ABC 

Laboratories, 

1983 

Acceptable 

Chronic: 

Early-life 

stage 

study 

Rainbow Trout 

(Oncorhyncus 

mykiss)

99.5 NOAEC/LOAEC: 

43.1/92.4 mg ae/L1

Larval 

survival 

41315804 

Ward, 1988 

Supplemental 

Fathead 

Minnow 

(Pimephales 

promelas) 

99.6 NOAEC/LOAEC: 

118/>118 mg ae/L 

No treatmentrelated 

effects 

45119711 

Drottar et al., 

1998 

Acceptable 

Chronic: 

Full life 

cycle 

Fathead 

Minnow 

(Pimephales 

promelas)

100 

NOAEC/LOAEC: 

120/>120 mg ae/L 

No treatmentrelated 

effects 

45119712 

Drottar et al., 

1999 

Supplemental 

Freshwater 

invertebrates 

Acute 

Waterflea 

(Daphnia 

magna) 

93 48-hour 

EC50: >100 mg/L 

Practically 

non-toxic 

00131632 

ABC 

Laboratories, 

1983 

Acceptable 

Chronic: 

Life cycle 

Waterflea 

(Daphnia 

magna)

99.5 21-day 

NOAEC/LOAEC: 

97.1/>97.1 mg/L 

No effects on 

growth or 

reproduction 

41315805 

Manning, 

1988 

Acceptable 
9 

Table 8. Summary of Endpoints from Aquatic Toxicity Studies for Imazapyr Acid

Taxonomic 

Group 

Study 

Type 

Species % 

ae

Endpoint Toxicity 

Category / 

Endpoint 

Affected

MRID No. 

Author/Year

Study 

Classification

Estuarine/ 

Marine Fish 

Acute 

Silverside 

minnow 

(Menidia 

menidia)

99.5 96-hour 

LC50: >184 mg 

ae/L 

Practically 

non-toxic 

41315801 

Manning , 

1988 

Acceptable 

Chronic* Data not 

submitted 

N/A N/A N/A N/A N/A 

Estuarine/ 

Marine 

Invertebrates 

Acute 

Eastern oyster 

(Crassostrea 

virginica) 

99.6 96-hour 

EC50 >132 mg 

ae/L 

Practically 

non- toxic 

45119710 

Drottar et al., 

1997 

Acceptable 

Eastern oyster 

(Crassostrea 

virginica) 

99.5 96-hour 

EC50: >173 mg 

ae/L 

Practically 

non- toxic 

41315802 

Ward, 1998 

Supplemental 

Pink shrimp 

(Penaeus 

duorarum)

99.5 LC50 >189 mg 

ae/L 

Practically 

non-toxic 

41315803 

Manning, 

1988 

Acceptable 

Chronic* Data not 

submitted 

N/A N/A N/A N/A N/A 

* The EC50/NOAEC values from the toxicity tests with the isopropylamine salt of imazapyr are expressed in acid 

equivalents (a.e.). The toxicity values with the acid are not expressed in terms of acid equivalents. 

1

Equation for calculating acid equivalents (a.e.): (molecular weight of acid / molecular weight of salt) x endpoint 

derived from study with isopropylamine salt as test compound. 

Therefore, given the endpoint from a study conducted with the salt, to obtain the acid equivalent of the 

concentration, multiply the endpoint in terms of a.i. of salt by 0.9098 (i.e., 293.16 g/mol divided by 322.19 g/mol) to 

obtain the concentration in acid equivalents. 

*Estuarine and Marine Fish and Invertebrates, Chronic

Chronic studies on estuarine/marine fish and invertebrates were not submitted. Although acute 

studies are available for freshwater and marine/estuarine fish and invertebrates and chronic 

studies are available for freshwater fish and invertebrates, acute to chronic ratios could not be 

estimated because all of the acute LC50/EC50 values are greater than values. As a result, an 

assumption will be made that the marine/estuarine fish and invertebrates will have a similar 

sensitivity to imazapyr as freshwater fish and invertebrates, especially since the acute LC50

values of these taxa were all greater than 100 mg a.e./L. There is uncertainty associated with this 

assumption because quantifiable taxonomic sensitivity factors between the two broad categories 

of fish and invertebrates do not exist. There is additional uncertainty associated with the 

estimated chronic NOAEC for estuarine/marine fish and for estuarine/marine invertebrates 

because the acute toxicity data do not allow for a definitive determination of the relative 

sensitivity of freshwater and estuarine/marine fish and invertebrates. 
10 

Effects to Aquatic Plants

Unlike the effect of the isopropylamine salt to aquatic animals with respect to the imazapyr acid 

(i.e. the salt is no more toxic than the acid), the toxicity of the isopropylamine salt is more toxic 

to aquatic vascular plants and non-vascular plants (based on duckweed and green algae) than 

imazapyr acid (Table 9). 

Table 9. Non-target Aquatic Plant Toxicity for Imazapyr Acid and Isopropylamine Salt of Imazapyr

Species 

[Tier II]

% ae EC50/NOAEC Endpoints 

Affected

MRID No. 

Author, Year

Study 

Classification

Isopropylamine Salt of Imazapyr*

Duckweed 

(Lemna gibba) 

23.3 0.018/0.011 

(mg ae/L) 

Frond production 43889102 

Hughes et al., 1995 

Acceptable 

Green Algae 

(Selenastrum capricornutum)

23.3 11.5/7.16 

(mg ae/L) 

Slight change in 

cell shape 

43889102 

Hughes et al., 1995 

Acceptable 

Imazapyr Acid

Duckweed 

(Lemna gibba) 

99.5 0.024/0.01 

(mg a.i./L) 

Population growth 

Frond production 

40811802 

Hughes, 1987 

Acceptable 

Green Algae 

(Selenastrum capricornutum) 

99.5 71/50.9 

(mg a.i./L) 

Cell density 40811802 

Hughes, 1987 

Acceptable 

Blue-green Algae 

(Anabaena flos-aquae) 

99.5 12.2/9.6 

(mg a.i./L) 

Cell density 40811802 

Hughes, 1987 

Acceptable 

Diatom 

(Navicula pelliculosa) 

99.5 >41/41 

(mg a.i./L) 

Cell density 40811802 

Hughes, 1987 

Acceptable 

Diatom 

(Skeletonema costatum) 

99.5 92/15.9 

(mg a.i./L) 

Cell density 40811802 

Hughes, 1987 

Acceptable 

*

The EC50/NOAEC values from the toxicity tests with the isopropylamine salt of imazapyr are expressed in acid 

equivalents (a.e.)

Terrestrial Effects Characterization

Terrestrial Animals

Table 10 provides a summary of endpoints from terrestrial toxicity studies for imazapyr. The 

available toxicity data (subacute dietary avian and rat acute studies) for the salt are no more toxic 

than the acid and are summarized in Appendix C. 
11 

Table 10. Summary of Endpoints from Terrestrial Animal Toxicity Studies for Imazapyr Acid

Taxonomic 

Group 

Study Type Species % 

ae

Endpoint Toxicity 

Category / 

Endpoint 

Affected

MRID No. 

Author/Year

Study 

Classification

Birds 

Acute Oral 

Northern 

bobwhite quail 

(Colinus 

virginianus)

93 LD50: >2,150 

mg ae/kg-bw 

Practically 

non-toxic 

00131633 

Bio-Life 

Assoc., 1983 

Acceptable 

Mallard duck 

(Anas 

platyrhynchos)

93 LD50: >2,150 

mg ae/kg-bw 

Practically 

non-toxic 

00131634 

Bio-Life 

Assoc., 1983 

Acceptable 

Sub-acute 

Dietary 

Northern 

bobwhite quail 

(Colinus 

virginianus)

93 8-day LC50: 

>5,000 mg ae/kgdiet 

Practically 

 non-toxic 

00131635 

Bio-Life 

Assoc., 1983 

Acceptable 

Mallard duck 

(Anas 

platyrhynchos)

93 8-day LC50: 

>5,000 mg ae/kgdiet 

Practically 

non-toxic 

00131636 

Bio-Life 

Assoc., 1983 

Acceptable 

Chronic: 

Reproduction 

Northern 

bobwhite quail 

(Colinus 

virginianus)

100 NOAEC/LOAEC: 

1,670/>1,670 

mg ae/kg-diet 

No treatmentrelated toxicity 45119714 

Ahmed et al., 

1999 

Acceptable 

Northern 

bobwhite quail 

(Colinus 

virginianus)

Tech NOAEC/LOAEC: 

2000/>2000 

mg ae/kg-diet 

No treatmentrelated toxicity 43831401 

1987 

Acceptable 

Mallard duck 

(Anas 

platyrhynchos)

Tech NOAEC/LOAEC: 

1890/>1890 

mg ae/kg-diet 

(2000 nominal) 

No treatmentrelated toxicity 43831402 

1987 

Acceptable 

Mammals 

Acute 

Rat 

(SpragueDawley) 

93 LD50 >5,000 

mg ae/kg bw 

(males/females) 

Mortality 00132030 

American 

Cyanamid 

Co., 1983

Acceptable

Chronic: 

Developmental2 

Rat 

(Sprague 

Dawley) 

93 

NOAEL/LOAEL 

= 300/1000 

mg/kg bw/day 

Maternal tox1 00131611 

Salamon & 

Mayhew, 

1983 

Acceptable 

NOAEL = 1000 

mg/kg bw/day Developmental

Rabbit 

(New Zealand 

White) 

93 

NOAEL = 400 

mg/kg bw/day No effects 

00131613 

Mayhew & 

Salamon, 

1983 

Acceptable 
12 

Table 10. Summary of Endpoints from Terrestrial Animal Toxicity Studies for Imazapyr Acid

Taxonomic 

Group 

Study Type Species % 

ae

Endpoint Toxicity 

Category / 

Endpoint 

Affected

MRID No. 

Author/Year

Study 

Classification

Chronic: 

Reproduction 

Rat 

(Sprague 

Dawley) 

99.5 

NOAEL = 738 

mg/kg bw/day - 

Males 

NOAEL = 933.3 

mg/kg bw/day – 

Females 

10000 ppm for 

both males and 

females. 

No effects 

41039505 

Robinson, 

1987 

Acceptable 

Terrestrial 

invertebrates Acute contact 

Honey Bee 

(Apis 

mellifera) 

Tech LD50: >100 

μg ae/bee 

Practically 

non-toxic 

00131637 

Atkins, 1983 

Acceptable

1

 Maternal toxicity - Gravid dams exhibited salivation during gestation days 8 - 15 (likely related to gavage route of administration). 2 Developmental toxicity - No treatment-related effects in developmental parameters; no treatment-related malformations 

Terrestrial Plants

Results of Tier II toxicity studies with monocots and dicots indicate that seedling emergence and 

vegetative vigor are severely impacted by exposure to imazapyr acid and to the isopropylamine 

salt of imazapyr (Table 11). 

Table 11. Summary of Endpoints from Terrestrial Plant Toxicity Studies for Imazapyr Acid and 

Isopropylamine Salt

Taxonomic 

Group 

Study 

Type 

Species Compound EC25 

lbs 

ae/A

NOAEC 

/[EC05]

** 

lbs ae/A 

Endpoint 

Affected 

MRID No. 

Author/Year

Study 

Classification

Monocot 

Seedling 

Emergence 

Oat 

Acid 

0.054 0.0156 Height 

40811801 

Banks, 1988 Supplemental 

Onion 0.034 [0.01] Weight 

Wheat 0.0046 [0.00099] Weight 

Dicots 

Sugar 

beet 

0.0024 [0.00017] Weight 

Tomato 0.008 0.0003 Weight 

Monocots 

Vegetative 

Vigor 

Corn >0.0156 0.0078 Weight 

Oat 0.013 0.0039 Height 

Wheat 0.012 0.0039 Weight 

Dicots 

Sugar 

beet 

0.00097 [0.00039] Weight 

Sunflower 0.0054 0.0039 Weight 

Cucumber 0.0009 [0.000064] Height 
13 

Table 11. Summary of Endpoints from Terrestrial Plant Toxicity Studies for Imazapyr Acid and 

Isopropylamine Salt

Taxonomic 

Group 

Study 

Type 

Species Compound EC25 

lbs 

ae/A

NOAEC 

/[EC05]

** 

lbs ae/A 

Endpoint 

Affected 

MRID No. 

Author/Year

Study 

Classification

Tomato >0.0156 0.00097 Weight 

Monocots Onion 

Salt* 

0.012 [0.005] Dry 

weight 43889101 

Feutz & 

Canez, 1995 Acceptable 

Dicots 

Soybean 0.034 0.008 Shoot 

length 

Sugar 

beet 

0.002 0.001 Dry 

weight 

*

The EC50/NOAEC values from the toxicity tests with the isopropylamine salt of imazapyr are expressed in acid equivalents (a.e.). **If the NOAEC value is above the EC25, equal to the EC25, or below the lowest concentration, an EC05 value is used instead. 

Bold values are used in risk assessment.

Ecological Incidents 

The ecological incident information system (EIIS) is an OPP database that houses ecological 

incidents that have been reported to the Agency. When available, EIIS includes date and 

location of an incident, type and magnitude of effects observed in various species, use(s) of 

pesticides known or suspected of contributing to the incident, and results of any chemical residue 

analysis or other analyses conducted during incident investigation. EIIS incidents are 

categorized according to the certainty that the incident resulted from pesticide exposure. The 

Avian Monitoring System (AIMS) is a database administered by the American Bird Conservancy 

that contains publicly available data on reported avian incidents involving pesticides. Many of 

the incidents listed in this database are also in the EIIS. However, there was only a single report 

in the AIMS database linked to the isopropylamine salt of imazapyr (PC Code 128829) which 

was considered an unlikely factor in the death of 70 birds from a fence row multiple herbicide 

use in South Carolina in 1992 (#I000022-001). 

In addition to the incidents recorded in EIIS and AIMS, the Agency maintains a database (IDS) 

which stores counts of minor incidents that pesticide registrants record in quarterly aggregated 

incident reports. Ecological incidents recorded in these aggregated reports include those 

categorized as ‘minor fish and wildlife’ (W-B), ‘minor plant’ (P-B), and ‘other non-target’ 

(ONT) incidents. ‘Other non-target’ incidents include reports of adverse effects to insects and 

other terrestrial invertebrates. A review of this database in February 2014 indicates the 

following: 

 A total of 229 incidents for the imazapyr acid (PC Code 128821): all 229 P-B. 

A total of 61 incidents for the isopropylamine salt (PC Code 128829): 6 W-B, 55 P-B. 

 

Twelve incidents resulting from imazapyr and its isopropylamine salt use have been recorded in 

the Ecological Incident Information System (EIIS) as of April 25, 2005. Incidents reported 

include possible impacts to terrestrial and aquatic plants, fish and birds. The majority of reported 

incidents are damage to terrestrial plants, especially food crops as a result of exposure following 
14 

application of formulations containing imazapyr and other active pesticide ingredients (Table 

12a and 12b). 

Table 12a. Incident Reports Involving Imazapyr Acid (04/20/1995 – 8/14/2013)

Location 

and Incident # 

Organism 

Involved 

Product Contact with 

Product/Symptoms Certainty Legality 

Driveway CO 

1006019-001 

Willow and 

Spruce 

Imazapyr Driveway surface 

application/Mortality 

Probable Intentional 

misuse 

Chelan County 

WA 

1014406-001 

Cherry and pear 

trees 

Imazapyr Applied in irrigation 

canal/Several dead 

or dying cherry and 

pear tress. 

Possible Unknown 

Dubuque 

County IA 

1008079-001 

Corn crop Mixture of 

Hornet 

(unknown 

pesticide) and 

Lightning 

(imazapyr) 

Post-emergent 

application/low yield 

and death in 120 

acres. 

Possible Registered Use 

Whitman 

County/ 

Washington 

State 

1014407-017 

Winter wheat Imazapyr Carryover residues 

from application to 

peas the previous 

Spring/damage. 

Complication 

assigning causes to 

the case concerning 

detection limits for 

imazapyr. 

Probable Unlikely 

Texas 

1005972-001 

Live oaks Imazapyr Application to 

neighbor’s 

property/injury to 3 

live oaks. 

Probable Registered Use 

Halifax County, 

NC 

1003826-008 

Goldfish Arsenal 

(imazapyr) 

Aerially applied to 

nearby 145 acre area 

followed by rain 12 

days later/mortality. 

No residues found in 

area. Cause of 

mortality not 

determined. 

Possible Registered Use 

Grant, WA 

1020459-003 Potato Imazapyr Plant damage due to drift exposure. Possible Unknown 

Montgomery, 

IN 

1012366-031 Corn Imazapyr 

Plant damage from 

direct broadcast 

application to 203 

acres of corn. 

Probable Misuse 

Grant, WA 

1013884-015 

Birch and 

Willow Imazapyr Mortality from spray drift. 

Highly 

Probable Misuse 

Limestone, AL 

1025684-001 Tomato Imazapyr Plant damage to 1,200 tomato plants Possible Unknown 
15 

from spray drift. 

Adams, WA 

1020459-012 Wheat Imazapyr Plant damage to wheat from runoff. Possible Unknown 

Grant, WA 

1020459-001 Wheat Imazapyr 

Plant damage to 

wheat from drift and 

runoff. 

Possible Unknown 

Bolivar, MO 

1025432-001 Green bean and 

Tomato Imazapyr 

Mortality to green 

beans and tomatoes 

from spray drift. 

Possible Registered Use 

Leake, MS 

1025563-001 Soybean Imazapyr Plant damage to 0.75 acres. 

Possible Unknown 

Maricopa, AZ 

1024071-369 Trees Imazapyr Mortality due to direct treatment. Possible Unknown 

Table 12b. Incident Reports Involving the Isopropylamine Salt of Imazapyr (05/26/1992 – 06/04/2004)

Location 

and Incident # 

Organism 

Involved 

Product Contact with 

Product/Symptoms Certainty Legality 

Aiken County, 

SC 

1000022-001 

Birds and Fish Arsenal 

(imazapyr) 

Karmex 

(Diuron) 

Escort 

(Metsulfuron 

methyl) 

Spray on fence row 

drifted onto adjacent 

bird nest boxes located 

from 2-85 feet of 

application site; runoff 

into a pond 60 foot 

away/bird kill of 

nesting and mature 

birds and fish and 

algae kill in pond. 

Possible Unknown 

Cass County, 

Texas 

1015265-001 

Loblolly pine 

seedlings 

Hexazinone, 

glyphosate, 

imazapyr 

Hexazinone in soil 

samples, Onestep 

(glyphosate and 

imazapyr) used in site 

preparation for 

planting of 

seedlings/mortality 

Possible Registered Use 

AR 

1015280-001 

Tomatoes, 

Cantaloupes 

Water-melons 

Krenite 

Arsenal 

Escort 

Glyphos 

(glyphosate, 

imazapyr, 

metsulfuron 

methyl) 

Krenite/Arsenal/Escort 

mixture applied to 

transmission right-ofway (150 yards to 1/4 

mile distance). 

Glyphos applied to 

pond levee 30-40 feet 

from watermelon and 

cantaloupe 

fields./Severe curling 

of oldest leaves on 

tomato plants with 

interveinal chlorosis. 

Mature tomatoes 

rotted from stem side. 

Watermelon and 

Possible Registered Use 
16 

cantaloupes aborted 

blooms and fruit. 

Pattern of glyphosate 

drift could be seen 

across field. No 

residues of any of 

applied pesticides 

found in plant tissues. 

Washington 

County Florida 

1013550-007 

Beans 

Oak Trees 

Grape vines 

Garlon 4 

Chopper 

Spraying to a forest 

site 150-200 feet 

away/beans exhibited 

chlorosis and cupping; 

grapes were chlorotic, 

and some oak leaves 

turned brown. 

Probable Registered Use 
17 

5. EXPOSURE PATHWAYS OF CONCERN 

The high persistence of imazapyr in terrestrial limits potential routes of dissipation to leaching, 

runoff, and spray drift. In aquatic environments, imazapyr dissipation is expected to be 

controlled by photodegradation. Field dissipation studies show that imazapyr is moderately 

persistent under typical use conditions. Although the routes of dissipation of imazapyr were not 

clearly defined in field dissipation studies, there was discernible movement or leaching of 

imazapyr into the soil profile. 

The exposure pathways for imazapyr may result in a wide range of potential aquatic and 

terrestrial exposure scenarios. For this problem formulation, the drinking water and inhalation 

pathways were screened using the SIP (Screening Imbibition Program) and STIR (Screening 

Tool for Inhalation Risk) screening methods and were found to be exposure pathways of low 

concern. However, it is noted that the screening analysis do not include aggregation with all 

other exposure pathways (dietary, dermal, inhalation, or drinking water), which together, may 

contribute to a total exposure that has a potential for effects to non-target animals. The risk 

characterization section will discuss the impact of the consideration of other routes of exposure 

that have been identified as potentially important, and the degree of certainty associated with 

screening-level risk assessment conclusions. SIP and STIR are described in detail at 

http://www.epa.gov/oppefed1/models/terrestrial/index.htm and the SIP and STIR model outputs 

are available in APPENDIX D and E, respectively. 

 

6. ANALYSIS PLAN 

Drinking Water Assessment 

EFED does not expect to conduct a new drinking water assessment given that the Health Effects 

Division (HED) does not anticipate conducting a dietary assessment. A drinking water 

assessment may be updated if a human health dietary risk assessments of imazapyr is conducted. 

The current models available for conducting drinking water assessment for surface water include 

PRZM (Pesticide Root Zone Model, version 3.12.3, June 2006) and EXAMS (Exposure Analysis 

Modeling System, version 2.98.04.06, April 2005) models. PRZM will be used to simulate 

pesticide transport as a result of runoff and erosion from an agricultural field while EXAMS will 

be used to estimate environmental fate and transport of pesticides in the index reservoir. 

Measures of exposure will be based on maximum single and year (including multiple seasons per 

year) labeled application rates, minimum retreatment intervals, and conservative methods of 

application. 

Currently, groundwater modeling is completed using PRZM-Groundwater (PRZM-GW version 

1.0, December 11, 2012), using the GW-GUI (Graphical User Interface, version 1.0, December 

11, 2012) and SCI-GROW (Screening Concentration in Groundwater, version 2.3, August 8, 

2003). SCI-GROW is an empirical model based on a linear best fit through 13 single-application 

groundwater studies. PRZM-GW can be used to simulate vertical pesticide transport through the 
18 

soil profile into an aquifer following application to an agricultural field. The PRZM scenarios 

used in groundwater simulations are representative of vulnerable sites—shallow unconfined 

aquifers under an agricultural field with leaching prone soil—and output values represent 

pesticide concentrations that may be observed in drinking water resulting from the use of 

groundwater vulnerable to pesticide contamination as source water. The results are expected to 

represent the upper bound values on the concentrations of toxic residues that might be found in 

drinking water supplied by groundwater. 

Ecological Risk Assessment 

Measures of Exposure 

In order to estimate risks of imazapyr exposure to aquatic and terrestrial organisms, all exposure 

modeling and resulting risk conclusions will be made based on maximum application rates, 

application methods, and any mitigation measures specifically indicated on the label. The models 

that will be used to predict the EECs of imazapyr are discussed on OPP’s model website.1

 These 

models include: PRZM (Pesticide Root Zone Model) and EXAMS (Exposure Analysis Modeling 

System), PRZM-groundwater (GW), AgDrift, AgDisp, T-REX, TerrPlant, SIP (Screening 

Imbibition Program), and STIR (Screening Tool for Inhalation Risk). Because imazapyr is 

applied through aerial and ground spray and is an herbicide, a comprehensive spray drift buffer 

zone analysis will be conducted. 

For imazapyr, the predicted estimated environmental concentrations of imazapyr acid will be 

used to calculate risk quotients. For the T-REX model, we will assume that the default half-life 

value of 35 days for foliar residue dissipation will capture the parent and degradates, unless data 

becomes available to suggest that the 35-day half-life is inadequate. If the estimated 

environmental concentrations (EECs) generated by T-REX with the default foliar dissipation 

half-life result in risk quotients (RQs) that are sufficient to exceed any levels of concern (LOCs), 

then the risk assessor will obtain more data or alter the default value to characterize risk with 

alternative values. (It should be noted that in T-REX, the foliar dissipation half-life does not 

affect EECs when only one application is simulated.) 

Measure of Effects 

Ecological effects data are used as measures of direct and indirect effects to biological receptors. 

Data are typically obtained from registrant-submitted studies or from literature studies identified 

by ECOTOX. The ECOTOX database provides more ecological effects data in an attempt to 

bridge existing data gaps. ECOTOX is a source for locating single chemical toxicity data and 

potential chemical mixture toxicity data for aquatic life, terrestrial plants, and wildlife. ECOTOX 

was created and is maintained by the U.S. EPA, Office of Research and Development, and the 

National Health and Environmental Effects Research Laboratory’s Mid-Continent Ecology 

Division. 

 

1 http://www.epa.gov/opp00001/science/models_pg.htm
19 

Integration of Exposure and Effects 

Risk characterization is the integration of exposure and ecological effects to determine the 

potential ecological risk from the use of pesticides and the likelihood of direct and indirect 

effects to non-target organisms in aquatic and terrestrial habitats. For the assessment of risks, the 

risk quotient (RQ) method is used to compare exposure and measured toxicity values. EECs are 

divided by acute and chronic toxicity values. The resulting RQs are then compared to the 

Agency’s Levels of Concern (LOCs) (USEPA 2004). These criteria will be used to indicate 

when imazapyr use, as directed on the label, has the potential to cause adverse direct or indirect 

effects to non-target organisms. In addition, any incident data from the EIIS, IDS, and AIMS will 

be considered as part of the risk characterization.

Endangered Species Assessments 

Consistent with the Agency’s responsibility under the Endangered Species Act (ESA), the 

Agency will evaluate exposure and potential risks to federally-listed threatened and/or 

endangered (listed) species from registered uses of pesticides in registration review. 

Endocrine Disruptor Screening Program 

As required by FIFRA and FFDCA, EPA reviews numerous studies to assess potential adverse 

outcomes from exposure to chemicals. Collectively, these studies include acute, subchronic and 

chronic toxicity, including assessments of carcinogenicity, neurotoxicity, developmental, 

reproductive, and general or systemic toxicity. These studies include endpoints which may be 

susceptible to endocrine influence, including effects on endocrine target organ histopathology, 

organ weights, estrus cyclicity, sexual maturation, fertility, pregnancy rates, reproductive loss, 

and sex ratios in offspring. For ecological hazard assessments, EPA evaluates acute tests and 

chronic studies that assess growth, developmental and reproductive effects in different 

taxonomic groups. As part of its most recent registration decision for imazapyr EPA reviewed 

these data and selected the most sensitive endpoints for relevant risk assessment scenarios from 

the existing hazard database. However, as required by FFDCA section 408(p), imazapyr is 

subject to the endocrine screening part of the Endocrine Disruptor Screening Program (EDSP). 

EPA has developed the EDSP to determine whether certain substances (including pesticide 

active and other ingredients) may have an effect in humans or wildlife similar to an effect 

produced by a “naturally occurring estrogen, or other such endocrine effects as the Administrator 

may designate.” The EDSP employs a two-tiered approach to making the statutorily required 

determinations. Tier 1 consists of a battery of 11 screening assays to identify the potential of a 

chemical substance to interact with the estrogen, androgen, or thyroid (E, A, or T) hormonal 

systems. Chemicals that go through Tier 1 screening and are found to have the potential to 

interact with E, A, or T hormonal systems will proceed to the next stage of the EDSP where EPA 

will determine which, if any, of the Tier 2 tests are necessary based on the available data. Tier 2 

testing is designed to identify any adverse endocrine-related effects caused by the substance, and 
20 

establish a dose-response relationship between the dose and the E, A, or T effect. 

Under FFDCA section 408(p), the Agency must screen all pesticide chemicals. Between 

October 2009 and February 2010, EPA issued test orders/data call-ins for the first group of 67 

chemicals, which contains 58 pesticide active ingredients and 9 inert ingredients. A second list 

of chemicals identified for EDSP screening was published on June 14, 20132 and includes some 

pesticides scheduled for registration review and chemicals found in water. Neither of these lists 

should be construed as a list of known or likely endocrine disruptors. 

For further information on the status of the EDSP, the policies and procedures, the lists of 

chemicals, future lists, the test guidelines and the Tier 1 screening battery, please visit our 

website.3

 

7. PRELIMINARY IDENTIFICATION OF DATA GAPS 

Table 13 identifies the environmental fate studies submitted by the registrant, as well as study 

classifications and whether or not additional data are needed in order to support the registration 

review exposure assessments. 

Table 13. Submitted Environmental Fate Data for Chemical Imazapyr and its Isopropylamine Salt 

Guideline Data Requirement 

(material) 

MRID # Study 

Classification 

Are data needed to conduct 

risk assessment? 

835.2120 

Hydrolysis 

(Imazapyra

) 00132359 Acceptable No 

835.2240 

Photodegradation in Water 

(Imazapyr) 00131617 Acceptable No 

835.2210 

Photodegradation on Soil 

(Imazapyr) 40003713 Acceptable No 

835.2370 Photodegradation in Air — — — 

835.4100 

Aerobic Soil Metabolism 

(Imazapyr) 

(Imazapyr) 

41023201 

45119701 

Acceptable 

Supplemental No 

835.4200 

Anaerobic Soil Metabolism 

(Imazapyr) 00131619 Acceptable No 

835.4400 

Anaerobic Aquatic 

Metabolism 

(Imazapyr) 

40003712 Acceptable 

No 

Aerobic Aquatic Metabolism 

(Imazapyr) 

(Photoproducts: CL 9140, CL 

119060) 

41002301 

41891501 

45119702 

Acceptable 

Supplemental 

Acceptable No 

 

2

 See http://www.regulations.gov/#!documentDetail;D=EPA-HQ-OPPT-2009-0477-0074 for the final second list of 

chemicals. 3 http://www.epa.gov/endo/
21 

Table 13. Submitted Environmental Fate Data for Chemical Imazapyr and its Isopropylamine Salt 

Guideline Data Requirement 

(material) 

MRID # Study 

Classification 

Are data needed to conduct 

risk assessment? 

835.4300 

(Photoproducts: CL 9140, CL 

119060) 

835.1230 

835.1240 

LeachingAdsorption/Desorption 

(Imazapyr + photoproducts) 

45119705 Acceptable 

Nob

835.1410 Laboratory Volatility — — — 

835.8100 Field Volatility — — — 

835.6100 

Terrestrial Field Dissipation 

(Imazapyr) 

(Arsenal 2ASa

) 

(Arsenal 2AS) 

45119706 

42192101 

42192102 

Supplemental 

Upgradeable 

Upgradeable No 

850.6100 ECM/ILV for Soil and Water 41891501 -- No 

835.6300 

Aquatic Field Dissipation 

(Imazapyr) 

(Arsenal 2AS) 

45119707 

41891501 

Unacceptable 

Supplemental No 

835,6400 Forestry Dissipation 

(Arsenal) 40003714 Acceptable No 

850.1730 Accumulation in Fish No 

850.1730 

Accumulation- aquatic nontarget 

(Imazapyr) 

(Imazapyr) 

45119707 

45119709 

Supplemental 

Supplemental 

No 

835.7100 Ground Water- small 

prospective 

— — — 

a

 Imazapyr refers to the parent compound (2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-

pyridinecarboxylic acid); Arsenal 2AS is the isopropylamine salt (2-Propanamine, 2-(4,5-dihydro-4-methyl-4-(1-methylethyl)-5-

oxo-1H-imidazol-2-yl]-3-pyridinecarboxylate). CL 9140 is pyridine 2,3-dicarboxylic acid. CL 119060 is Furo[3,4]pyridine5(7H), one-7-hydroxy. b Limited to two soils.

Ecological Effects 

Table 14 and Table 15 identify the ecological effects studies (aquatic and terrestrial, 

respectively) submitted by the registrant, as well as study classifications and whether or not 

additional data are needed in order to support the registration review risk assessments. 

A passerine study is not available; however, given the low sensitivity of the quail and mallard 

duck to imazapyr, it is assumed that further testing on passerines will not be of great value to 

inform on the potential toxicity of imazapyr. 
22 

The terrestrial plant guidelines (850.4100 and 850.4150) are partially fulfilled (four species of 

monocots, including corn; five species of dicots, including soybean and sugar beet) (MRID 

40811801; acid). 

Table 14. Submitted Aquatic Toxicity Data for Chemical Imazapyr and its Isopropylamine Salt 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

[Acid/Salt] 

Study 

Classifications 

[Acid/Salt] 

Are data needed 

to conduct risk 

assessment? 

Comments 

850.1010 

Freshwater 

invertebrate 

acute toxicity 

00131632 

00147117 

All acceptable No 

Although TEP data would address some 

uncertainty regarding potential toxicity of 

the formulations applied directly to water, 

the TGAI data show that imazapyr is 

practically non-toxic and no aquatic 

animal incidents have been reported. 

850.1025 

850.1035 

850.1045 

850.1055 

Saltwater 

invertebrate 

acute toxicity 

41315803 

41315802 

45119710 

No salt 

Acceptable 

Supplemental 

Acceptable 

No 

Although TEP data would address some 

uncertainty regarding potential toxicity of 

the formulations applied directly to water, 

the TGAI data show that imazapyr is 

practically non-toxic and no aquatic 

animal incidents have been reported. 

850.1075 

Freshwater fish 

acute toxicity 

00131630 

00131629 

00131631 

45119713 

00147116 

All acceptable No 

Although TEP data would address some 

uncertainty regarding potential toxicity of 

the formulations applied directly to water, 

the TGAI data show that imazapyr is 

practically non-toxic and no aquatic 

animal incidents have been reported. 

850.1075 

Saltwater fish 

acute toxicity 

41315801 

No salt Acceptable No 

Although TEP data would address some 

uncertainty regarding potential toxicity of 

the formulations applied directly to water, 

the TGAI data show that imazapyr is 

practically non-toxic and no aquatic 

animal incidents have been reported. 

850.1300 Freshwater 

invertebrate 

life cycle 

41315805 

No salt Acceptable No -- 

850.1350 Saltwater 

invertebrates 

life cycle 

No data N/A No -- 

850.1400 Freshwater fish 

early-life stage 

41315804 

45119711 

No salt 

Supplemental 

Acceptable No -- 
23 

Table 14. Submitted Aquatic Toxicity Data for Chemical Imazapyr and its Isopropylamine Salt 

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

[Acid/Salt] 

Study 

Classifications 

[Acid/Salt] 

Are data needed 

to conduct risk 

assessment? 

Comments 

850.1400 

Saltwater fish 

early-life stage No data N/A No 

While data are unavailable for this taxa, 

sensitivity of saltwater fish is assumed to 

be similar to that of the freshwater fish, 

which in this case is low. 

850.1500 Freshwater fish 

full life cycle 

45119712 

No salt Supplemental No -- 

850.4400 

Aquatic plant 

growth: 

Tier II vascular 

plants 

40811802 

43889102 All acceptable No -- 

850.5400 

Aquatic Plant 

Growth: 

Tier II algae 

40811802 

43889102 All acceptable No -- 
24 

Table 15. Submitted Terrestrial Toxicity Data for Chemical Imazapyr and its Isopropylamine Salt

OCSPP 

Guideline 

Data 

Requirement 

Submitted 

Studies 

(MRID) 

[Acid/Salt] 

Study 

Classifications 

[Acid/Salt] 

Are data needed 

to conduct risk 

assessment? 

Comments 

850.2100 Avian Oral 

Toxicity 

00131633 

00131634 

No salt 

All acceptable No 

While data are unavailable for passerine 

(40 CFR, Part 158-Subpart G). acute oral 

studies on quail and mallard duck 

indicates low sensitivity to imazapyr. 

850.2200 Avian Dietary 

Toxicity 

00131635 

00131636 

00147115 

All acceptable No -- 

850.2400 

Wild 

Mammal 

Toxicity 

00132030 

00147049 

44735301 

All acceptable No -- 

850.3020 

Honeybee 

Acute Contact 

Toxicity 

00131637 

No salt Acceptable No -- 

850.2300 

Avian 

Reproduction 

Toxicity 

45119714a 

43831401 

43831402 

No salt 

Acceptable 

Acceptable 

Acceptable No -- 

850.4100 

Seedling 

Emergence 

Tier II 

40811801 

No salt 

Supplemental No -- 

850.4150 Vegetative 

Vigor Tier II 

40811801 

40003711 

43889101 

Supplemental 

Supplemental 

Acceptable 

No -- 

850.1735 

Whole 

sediment: 

acute 

freshwater 

invertebrates 

No data N/A No 

Half-life of the imazapyr in sediment is 

>10 days (aerobic aquatic metabolism 

study, MRID 41002301) but the log Pow is 

0.22. Furthermore, imazapyr is not 

expected to partition to sediment. 

850.1740 

Whole 

sediment: 

acute marine 

invertebrates 

No data N/A No 

Half-life of imazapyr in sediment is >10 

days (aerobic aquatic metabolism study, 

MRID 41002301), but the log Pow is 0.22. 

Furthermore, imazapyr is not expected to 

partition to sediment. 

Non-GLN 

Whole 

sediment: 

chronic 

invertebrates 

freshwater 

and marine 

No data N/A No 

Half-life of the imazapyr in sediment is 

>10 days (aerobic aquatic metabolism 

study, MRID 41002301), but the log Pow

is 0.22. Furthermore, imazapyr is not 

expected to partition to sediment. 

 
25 

APPENDIX A- Summary of Environmental Fate Data

Hydrolysis (835.2120, MRID 00132359, Study Status: Acceptable). Imazapyr is 

stable to hydrolysis at environmentally relevant pH values and temperatures. Imazapyr was 

stable in aqueous buffer solutions (pH 5 and 7) and distilled water (pH 5.2) for up to 30 days at 

25±1C. Imazapyr degraded slowly in an aqueous pH 9 buffer solution. The only identified 

degradate was 2-[(1-carbomyl-1,2-dimethylpropyl)carbomyl] nicotinic acid (CL 252974). Minor 

degradates were not identified. 

Aqueous Photolysis (835.2240, MRID 00131617, Study Status: Acceptable). 

Imazapyr is susceptible to photolysis. Imazapyr degraded with a half-life of 2.5-5.3 days in 

distilled water or buffer solution at pH 5 or 9, at 25C, irradiation with a xenon arc lamp (12 

hours/day) for up to 10 days. Two major degradates were formed: 2,3-pyridinecarboxylic acid 

(CL 9140, 22.7%) and 7-hydroxy-furo[3,4-b]pyridin-5(7H)-one (CL 119060, 9.7%). 

Unidentified degradates comprised up to 7.1% of the applied radioactivity. Imazapyr was stable 

in the dark during the 10-day incubation period. 

Soil Photolysis (835.2210, MRID 40003712, Study Status: Acceptable). Imazapyr 

degraded slowly on irradiated soil. 14C-pyridine-ring-labeled imazapyr applied at a rate of 1.5 lb 

ae/acre, degraded with an extrapolated half-life of 149 days on sandy loam soil irradiated a 4-

week period. No degradate was >10% of the applied radioactivity. In dark controls, imazapyr 

was 95-98% of the applied radioactivity after 4 weeks. Unidentified residues were 2.9-4.9% of 

the applied radioactivity. 

Aerobic Soil Metabolism (835.4100, MRID 41023201, Study Status: Acceptable). 

Imazapyr is essentially stable to degradation in soil maintained under aerobic conditions. 

14C/13C-labeled imazapyr was added to Princeton sandy loam soil (soil moisture 75% of field 

capacity) at a concentration of 1.5 ppm. Samples were maintained at 25C in the dark for up to 

365 days. Volatiles and/or CO2 were collected in traps. At 365 days, 88% of the applied 

radioactivity remained as parent imazapyr (calculated half-life of 5.9 years). Unextractable 

residues averaged 4% of the applied radioactivity during the study and accounted for 5% at 365 

days. Cumulative 14CO2 accounted for up to 7% of the applied radioactivity after 365 days. 

Material balance averaged 101%. 

Aerobic Soil Metabolism (835.4100, MRID 45119701, Study Status: Supplemental). 

The aerobic soil metabolism of [pyridine-6-14C]-imazapyr was studied for 121 days in Sassafras 

sandy loam soil (pH 6.0, organic matter 2.0%; soil moisture content of 75% of 1/3 bar) from 

New Jersey. Samples were keep in the dark at 25±1C. [14C]Imazapyr was applied at the 

nominal rate of 0.22 mg ae./kg soil (equivalent to 0.44 lb ae/acre or 0.50 kg ae/ha with 15-cm 

soil incorporation). Because the 121-day study period was short compared to the persistence of 

[pyridine-6-14C]imazapyr, there was insufficient time for the full pattern of formation and 

decline of products to develop. At study end, approximately 72% of the amount of parent 

recovered at the beginning of the study remained. A structurally similar transformation product, 
26 

2-[4-Isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl]-3-hydroxy pyridine, was present at 

approximately 7% of parent radioactivity recovered at the beginning of the study and increased 

slowly for the duration of the study. Volatilized 14CO2 totaled approximately 6% of initial parent 

radioactivity at 121 days. 

Two additional minor transformation products, 2-[(1-carbamoyl-1,2-dimethylpropyl)carbamoyl] 

nicotinic acid and 2-[4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl]-3-carboxymethyl pyridine, 

were each detected at 3% of the applied, but may have been artifacts produced during sample 

extraction and preparation. Organic volatiles were <0.1% of applied at any sampling interval. 

Material balance ranged from approximately 100% to 97%. 

The extrapolated first-order kinetics regression half-life for parent imazapyr for the 121-day 

period is 296 days (r2

 = 0.89, 95% confidence interval of 247 to 370 days). If the two putative 

artifact compounds are assumed to be parent material and included in the regression, then the 

apparent half-life increases slightly to 313 days (r2

 = 0.90, 95% confidence interval of 264 to 

387 days). 

Anaerobic Soil Metabolism (835.4200, MRID 00131619, Study Status: Acceptable). 

Imazapyr was stable in sandy loam soil. Sandy loam soil (1.8% organic matter, pH 5.3-6.5, CEC 

8.5 meq/100 g) was treated with a 50:50 mixture of 14C and 13C-carboxy-labeled imazapyr at 1 lb 

ae/acre. Radioactivity in the water extract was three-fold that in the soil. 

Anaerobic Aquatic Degradation (835.4400, MRID 40003712, Study Status: 

Acceptable). Imazapyr is resistant to anaerobic aquatic degradation. 14C-pyridine-labeled 

imazapyr, applied at 1.5 lb/A, was incubated in an anaerobic sand sediment:water system in the 

dark at 19-22C for up to 4 months. Imazapyr comprised 96-98% of the recovered radioactivity 

at all sampling intervals. 

Aerobic Aquatic Metabolism (835.4300, MRID 41002301, Study Status: Acceptable). 

Imazapyr did not degrade in water or sediment under aquatic conditions. 14C-Imazapyr was 

added to a series of water and soil sediments (92% sand) collected from an irrigation pond at a 

rate equivalent to 1.5 lb/A. Sample containers were maintained under positive pressure in the 

dark at 25C. 14CO2 accounted for <2% of the applied radioactivity 4 weeks (there were no 

organic volatiles). Over the course of the study, >96% of the applied radioactivity remained in 

the aqueous phase while <2% was bound to soil. Analysis by thin-layer chromatography 

determined that parent imazapyr accounted for >97% of the applied radioactivity after 4 weeks. 

Aerobic Aquatic Metabolism of Two Degradates of Imazapyr (835.4300, MRID 

45119702, Study Status: Acceptable). The aerobic degradation of the imazapyr degradation 

products, [pyridine-6-14C]-labeled furo[3,4-b]pyridine-5-(7H)-one-7-hydroxy (CL 119060) and 

pyridine 2,3-dicarboxylic acid (CL 9140), was studied in each of two pond water:sediment 

systems. A pond water:sediment system (water pH 8.2; sand sediment pH 7.7, organic matter 

0.8%) from Florida, and a pond water:sediment system (water pH 7.9; silt loam sediment pH 6.6, 

organic matter 1.1%) from Missouri were studied for 14 days in darkness at 25±1C. 
27 

[

14C]furo[3,4-b]pyridin-5(7H)-one,7-hydroxy- and [14C]pyridine 2,3-dicarboxylic acid was 

applied separately at the nominal rate of 0.083 mg ae/L. The sediment:water ratio used was 175 

g wet sediment:200 mL water. For both test compounds and systems, aerobic conditions were 

maintained in the water layers of the sediment:water systems, but the sediment layers remained 

anaerobic throughout the study. Levels of parent [14C]furo[3,4-b]pyridin-5(7H)-one,7-hydroxyand [14C]pyridine 2,3-dicarboxylic acid and transformation products were identified only in the 

water layers. To compensate for the failure to identify and quantify residues extracted from the 

sediments and low material balances during later sampling intervals, it was assumed that all 

extracted sediment residues were unreacted starting materials. These extracted sediment residues 

were then added to parent materials identified and quantified in the water phases to estimate total 

system (water + sediment phases) half-lives. 

 In the water layer, the major nonvolatile transformation product of [pyridine-6-14C]furo[3,4-

b]pyridin-5(7H)-one,7-hydroxy-pyridine 2,3-dicarboxylic acid. This compound was detected at 

maximums of 20% of applied at 1 day and 28% at 3 days, in the sand and silt loam sediment 

systems, respectively. Nicotinic acid was identified as a nonvolatile transformation product, and 

detected at maximums of 6% at 2 days and 10% at 1 day, in the sand and silt loam sediment 

systems, respectively. Unidentified polar products increased to maximums of 33-41% of applied 

at 7 days in both systems. For both test systems, nonextractable [14C]residues were 6% at any 

sampling interval. 14CO2 was a major volatile transformation product, totaling 45% for sand 

sediment:water systems. The first-order kinetics regression half-life for [pyridine-6-

14C]furo[3,4-b]pyridin-5(7H)-one,7-hydroxy- in the Florida pond system (water + sediment) was 

3.9 days (r2

 = 0.95, 95% confidence interval of 3.4 to 4.5 days). The Missouri pond system firstorder regression half-life (water + sediment) for [pyridine-6-14C]furo[3,4-b]pyridin-5(7H)-one,7-

hydroxy- was 5.8 days (r2

 = 0.80, 95% confidence interval of 4.5 to 8.2 days). The average halflife for the Florida and Missouri systems for [pyridine-6-14C]furo[3,4-b]pyridin-5(7H)-one,7-

hydroxy- was 5±1 days with an upper 90% confidence bound on the mean of 8 days. 

No major nonvolatile transformation products of [pyridine-6-14C]pyridine 2,3-dicarboxylic acid 

were detected in the water layers. Minor nonvolatile transformation products were unidentified 

polar products. For both sediments, nonextractable [14C]residues were 3.9% at any sampling 

interval. 14CO2 was a major volatile transformation product, totaling 53% for sand 

sediment:water systems and 23% for silt loam sediment:water systems at study termination. 

Organic volatiles were <0.1% of applied at any sampling interval. The first-order kinetics 

regression half-life for of [pyridine-6-14C]pyridine 2,3-dicarboxylic acid for the Florida pond 

system (water + sediment) was 3 days (r2

 = 0.56, 95% confidence interval of 1.9 to 5.9 days). 

The Missouri pond system first-order regression half-life (water + sediment) for pyridine 2,3-

dicarboxylic acid was 4 days (r2

 = 0.88, 95% confidence interval of 3.5 to 5.5 days). The 

average half-life for the Florida and Missouri systems for pyridine 2,3-dicarboxylic acid is 4±1.0 

days with an upper 90% confidence bound on the mean of 5.7 days. 
28 

Aerobic Aquatic Metabolism of Imazapyr Photodegradation Products (835.4300, 

MRID 41891501, Study Status: Supplemental). This study addressed the aerobic aquatic 

metabolism of the two major aqueous photodegradation products of imazapyr, 2,3-

pyridinedicarboxylic acid (CL 9140) and 7-hydroxy-furo[3,4-b]pyridin-5(7H)-one (CL 119060) 

over a 4-week period. The photoproducts were generated by irradiating 10 mg of imazapyr in 50 

mL of deionized water which yielded the major products CL 9140 and CL 119060 (22% and 

24%, respectively). The photoproducts were added to pond water and sediment from a wetland 

in New Jersey. Evolved CO2 was trapped and measured. The study presented evidence that CL 

119060 is rapidly converted to CL 9140) which, in turn, is metabolized to CO2. 

Adsorption/Desorption (835.1230/835.1240, MRID 45119705, Study Status: 

Acceptable). The batch-equilibrium adsorption/desorption characteristics of 14C-6-pyridinering-labeled- imazapyr and the metabolites (photoproducts) pyridine 2,3-dicarboxylic acid and 

furo[3,4-b]pyridin-5(7H)-one,7-hydroxy- (both 14C-6-labeled in the pyridine ring) were each 

studied separately in a sand sediment from Florida [pH - 7.7, organic carbon - 0.47%] and a silt 

loam sediment from Missouri [pH - 6.6, organic carbon - 0.64%]. The equilibrating solution 

used was 0.01 M CaCl2, at a soil/solution ratios of 1:2 (w:v). The desorption phase of the study 

was carried out at 20±1C for 27.5 hours (imazapyr), 24 hours (pyridine 2,3-dicarboxylic acid) 

or 44 hours (furo[3,4-b]pyridin-5(7H)-one,7-hydroxy-). The desorption phase was conducted 

once. Supernatant solutions were analyzed using LSC and HPLC. Sediments were extracted 

with 0.1 M sodium hydroxide following desorption, analyzed by LSC. Radioactivity in the soil 

residue after extraction was determined by combustion. 

Imazapyr: Freundlich adsorption parameters and average simple adsorption/desorption 

coefficients are given in Table A-1 below. The Freundlich adsorption coefficients (Kf) were 

0.091 (1/N=0.733; Kfoc= 19.4) and 0.523 (1/N=0.887; Kfoc=81.4) for the sand and silt loam 

sediments, respectively. Corresponding simple adsorption coefficients adjusted for organic 

carbon (Koc) were 30.6 and 99.8 mL/g organic carbon. 

Pyridine 2,3-dicarboxylic acid: Freundlich adsorption parameters and average simple 

adsorption/desorption coefficients are given in the results synopsis and in Table A-2 below. The 

Freundlich adsorption coefficients (Kf) were 0.699 (1/N=0.811; Kfoc= 149.4) and 31.6 

(1/N=0.955; Kfoc=4940) for the sand and silt loam sediments, respectively. Corresponding Koc

values were 217 and 6053 mL/g organic carbon. 

Furo[3,4-b]pyridin-5(7H)-one,7-hydroxy-: Freundlich adsorption parameters and average simple 

adsorption/desorption coefficients are given in the results synopsis and in Table A-3 below. The 

Freundlich adsorption coefficients (Kf) were 0.551 (1/N=0.939; Kfoc= 117) and 5.77 (1/N=0.962; 

Kfoc=902) for the sand and silt loam sediments, respectively. Corresponding Koc values were 134 

and 1020 mL/g organic carbon. 

 
29 

Table B-1: Adsorption and desorption parameters for imazapyr in the sediments.

Soil Freundlich Adsorption Simple Ads/Des Coefficient Averages 

Kf ads 1/N R2

 Kf ads oc Kads Kads oc Kdes Kdes oc 

Sand sediment (FL) 0.091 0.733 0.976 19.4 0.144 30.6 0.338 71.9 

Silt loam sediment (MO) 0.523 0.887 0.999 81.7 0.639 99.8 1.02 159 

Kf ads – Freundlich adsorption coefficient; 1/N – Slope of Freundlich adsorption isotherm. 

Subscript oc indicates sorption per unit organic carbon ([K x 100]/% organic carbon). 

R2

 - Regression coefficient of Freundlich equation.

Table B-2: Adsorption and desorption parameters for Pyridine 2,3-dicarboxylic acid in the 

sediments.

Soil Freundlich Adsorption Simple Ads/Des Coefficient Averages 

Kf ads 1/N R2

 Kf ads oc Kads Kads oc Kdes Kdes oc 

Sand sediment (FL) 0.699 0.811 0.998 149 1.02 217 1.60 340 

Silt loam sediment (MO) 31.6 0.955 0.999 4940 38.7 6053 140 21,900 

Kf ads – Freundlich adsorption coefficient; 1/N – Slope of Freundlich adsorption isotherm. 

Subscript oc indicates sorption per unit organic carbon ([K x 100]/% organic carbon). 

R2

 - Regression coefficient of Freundlich equation.

Table B-3: Adsorption and desorption parameters for Furo[3,4-b]pyridin-5(7H)-one,7-

hydroxy- in the sediments.

Soil Freundlich Adsorption Simple Ads/Des Coefficient Averages 

Kf ads 1/N R2

 Kf ads oc Kads Kads oc Kdes Kdes oc 

Sand sediment (FL) 0.551 0.939 0.998 117 0.628 134 1.89 402 

Silt loam sediment (MO) 5.77 0.962 1.000 902 6.53 1020 16.9 2640 

Kf ads - Freundlich adsorption coefficient; 1/N -Slope of Freundlich adsorption isotherm. 

Subscript oc indicates sorption per unit organic carbon ([K x 100]/% organic carbon). 

R2

 - Regression coefficient of Freundlich equation.

Terrestrial Field Dissipation (835.6100, MRID 42192101, Study Status: 

(Supplemental). Arsenal 2AS, the isopropylamine salt of imazapyr (22.6% ae in aqueous 

solution) dissipated with a calculated half-life of 143 days. However, the dissipation pathway(s) 

was not addressed. The test material was applied by boom sprayer to a bareground plot of silt 

loam soil (2.6% organic matter, pH 5.1, CEC 16.9 meq/100 g) located in Hillsboro, Oregon. 

Application rate was 1.53 lb ae/acre. Sprinkler irrigation was applied at 0.5 to 3.0 inches, 1-3 

times/month. The temperature ranged from 6 to 100F, total precipitation was 67 inches, and 

irrigation total was 29 inches. Imazapyr was found primarily in the 0-6-inch soil layer at a 

maximum concentration of 400 ppb. In the 6-12-inch layer, residues were 6 ppb at 1-30 days, 
30 

14-17 ppb at 240 days, and 6-13 ppb at 452 days. In the deeper soil layers residues were <8 ppb 

immediately posttreatment, and not detected at later sampling times. A half-life of 143 days was 

calculated. Residues in a control plot were 4 ppb. Routes of dissipation were not provided. 

Terrestrial Field Dissipation (835.6100, MRID 42192102, Study Status: 

Supplemental). Arsenal 2AS, the isopropylamine salt of imazapyr (22.6% ae in aqueous 

solution) dissipated with a calculated half-life of 64 days. However, the mode(s) of dissipation 

was not addressed. The test material was applied by backpack sprayer to a bareground plot of 

sandy loam soil (2.3% organic matter, pH 5.7, CEC 6.75 meq/100 g) located in Janesville, North 

Carolina. Application rate was 1.53 lb ae/acre. Sprinkler irrigation was applied at 0.58 to 2.0 

inches, 3-5 times/month. The temperature ranged from 18 to 98F, total precipitation was 67 

inches and irrigation totaled 22 inches. Imazapyr residues were found primarily in the 0-6-inch 

soil layer, averaging 300-440 ppb immediately posttreatment and declining to 7-12 ppb at 240-

360 days. In the 6-12-inch layer, residues were at a maximum of 10-21 ppb at 14 days. In the 

12-18 inch and 18-24 inch depths, residues reached a maximum of 8-14 ppb and 7-11 ppb, 

respectively, at 30 days. In the deeper soil layers, residues were 6 ppb at all sampling times. 

Residues in a control plot were 3 ppb. Routes of dissipation were not provided. 

Terrestrial Field Dissipation (835.6100, MRID 45119706, Study Status: 

Supplemental). This study shows that imazapyr is prone to leach and is relatively long-lived. 

Cropped corn fields in Iowa and Nebraska were used as the study sites. Apparent, simple firstorder regression half-lives at all sampled depths (0-36 inches) were 94 days in Iowa (95% 

confidence limits 84-107 days, r2

 = 0.88) and 126 days in Nebraska (95% confidence limits 113-

143 days, r2

 = 0.88). The degree to which the apparent loss/dispersal of imazapyr was due to 

transformation, plant uptake, volatilization, leaching, runoff, etc. was not determined. There was 

no analysis for transformation products (degradates or metabolites), no measurement of plant 

uptake, or attempt to correlate daily rainfall/irrigation with pesticide disappearance or potential 

for movement in soil during the course of study. Therefore, disappearance half-lives represent 

only lower limits for what may be much longer effective half-lives, and cannot be assumed to be 

attributable to degradation or detoxification of parent (or any possible by-products). 

For the present study, casual inspection of the soil concentration data shows quick infiltration of 

imazapyr into moist surface soil, and close correlation between rainfall/irrigation events with 

eventual leaching of imazapyr into the lower soil depths (down to 30 or 36 inches). 

Rainfall/irrigation was said to be typical or average for the experimental sites in Iowa and 

Nebraska. These penetrations into soil are evidence of high mobility. Furthermore, the leaching 

occurred in soils with characteristics that are not usually associated with high potential for 

leaching. The soils had relatively high cation exchange capacities, organic matter, and moisture 

retention capacity. Subsoil clay content was approximately 25-30%. 

Aquatic Field Dissipation (835.6300, MRID 41891501, Study Status: Supplemental). 

Arsenal 2AS rapidly dissipated from shallow ponds in Florida and Louisiana during summer 

months. Arsenal 2AS applied at the proposed maximum label rate of 1.5 lb ae/acre dissipated 
31 

with half-lives of 3 and 4 days in pond water and sediment, respectively. In a similar study 

carried out in Louisiana, Arsenal 2AS dissipated with half-lives of 2 and 4 days in pond water 

and sediment, respectively. The analytical detection limits were too high to provide an 

understanding of the formation and decline kinetics of the degradation kinetics. Because these 

studies were conducted in shallow ponds during the summer months, the rapid dissipation 

observed in these studies may not be representative of dissipation rates at other aquatic 

environments. 

Forestry Dissipation (835.6400, MRID 40003714, Study Status: Supplemental). 

Imazapyr applied by aerial spray to a forest dissipated primarily by runoff, and to a lesser degree, 

by foliar absorption. Arsenal was applied by helicopter at 2.24 kg/ha (2 lb ae/acre) with Igepal 

DM-710 as a surfactant, over two forested watersheds in Fayette and Randolph counties, 

Alabama, in May and June, 1985. Each site was divided into two water sheds, one treated and 

one control. Precipitation was measured at each site, and stream water levels were monitored. 

Samples were taken of water, suspended sediment, vegetation (composited), litter, and soil (up to 

20 inches depth), the latter by area of ridge, mid-slope and lower slope. 

Description, Fayette county site. This was a 121-ha site of mixed hardwoods. Pines in the plot 

had been harvested earlier. The application took place over 4 days due to weather conditions of 

intermittent fog and drizzle. The plot was drained by a single stream. The soil was sand loam 

(1.92% organic matter, CEC 3.7 meq/100 g). 

Description, Randolph county site. This was a 40-ha area of loblolly pine seedlings and mixed 

hardwoods. The plot was drained by a single stream. The underlying soil was loam (4.4% 

organic matter, CEC 4.17 meq.100 g). 

Results, Fayette county. Imazapyr dissipated from vegetation with a half-life of 12 days (highest 

mean residue, about 100 ppb at 3 days post-treatment), from litter with a half-life of 44 days, 

from 0-4-inch depth of bare ground soil with a half-life of 24 days, and from 0-4-inch depth 

litter-covered soil with a calculated half-life of 19 days. Residues were found primarily in the 0-

12-inch soil depth. Residues in stream grab samples were highest during application (up to 680 

ppb) and following storm events. Stream residues declined to trace amounts at later sampling 

intervals. Only one stream sediment sample contained a detectable residue, 52 ppb. 

Results, Randolph county. Imazapyr dissipated from vegetation with a half-life of 40 days 

(highest mean residue, 122 ppb immediately post-treatment), from litter with a half-life of 37 

days, from 0-4-inch depth of bare ground soil with a half-life of 26 days, and from 0-4-inch 

depth litter-covered soil with a calculated half-life of 34 days. Residues were found primarily in 

the 0-12-inch soil depth. Visual observation indicated at least 99% kill of all targeted vegetation. 

Bioconcentration in Aquatic Non-target Organisms (850.1730, MRID 45119707. 

Study Status: Supplemental). Parent imazapyr did not bioconcentrate appreciably in the fish 

and crayfish species tested (three fish and one crayfish species at each site, total of four different 

species). There were no tests for degradates in any of the test species. It should be noted that the 
32 

reported limit of quantitation for parent in tissue was a relatively high 50 ppb. In several 

instances, for unknown or unverified reasons, there was some limited mortality of some test 

species and/or their partial disappearance, such that for some sampling intervals there were 

insufficient amounts of tissue for analysis. However, this does not significantly alter the general 

conclusion of no appreciable bioconcentration of parent above a concentration of 50 ppb. 

 Bioconcentration in Aquatic Non-target Organisms (850.1730, MRID 45119709, 

Study Status: Supplemental). Under test conditions (28-day exposure periods at mean 

measured concentrations of 250 ppb for oysters and 260 ppb for shrimp followed by 14-day 

depuration periods), overall results for the two test species, oyster and shrimp, are essentially the 

same. There was no bioconcentration of pyridyl-6-14C-imazapyr [bioconcentration factor (BCF) 

<1)]. The study was deficient for both oyster and shrimp because of relatively high levels of 

quantitation (LOQ; 72 ppb for oyster, 128 ppb for shrimp) compared to exposure concentrations. 

Consequently, because of the relatively low and variable concentrations of residues compared to 

the LOQ, uptake and depuration rates of imazapyr could not be calculated meaningfully, and no 

metabolite identification work was conducted. The information was considered supplemental, 

but no additional studies are needed. 
APPENDIX B-Transformation Products in Environmental Fate Studies 

Study MRID 

Study Type System 

Imazapyr 

half-life 

Maximum transformation products 

CL 

288247 1

CL 252974 CL 119060 

00132359 Hydrolysis (161-1) pH 5 at 25C Stable ND2 ND ND 

pH 7 at 25C Stable ND ND ND 

pH 9 at 25C Stable ND 6.9 ND 

00131617 Photolysis in water (161-2) pH 5 and 9 at 25C 

(12 hour exposure cycle) 

2.5 - 5.3 days ND ND 9.7 

40003713 Photolysis in soil (161-3) Loamy sand soil Stable 

 (~149 days) 

ND ND ND 

41023201 Aerobic Soil Metabolism (162-1) Loamy sand soil Stable ND ND ND 

45119701 Aerobic Soil Metabolism (162-1) (Supplemental) Loamy sand soil (~5.9 years) 

>296 days ND 3 ND 

00131619 Anaerobic Soil Metabolism (162-2) Loamy sand soil Stable (>60 days) ND ND ND 

40003712 Anaerobic Aquatic Metabolism (162-3) Total system >120 days ND ND ND 

41002301 45119702

Aerobic Aquatic Metabolism (162-4) Aerobic Aquatic Metabolism (162-4) - Degradate metabolism 

Total system Total system (CL 119060 metabolism) (CL 9140 metabolism) 

>120 days 4.9 days 3.6 days 

ND NA NA 

ND NA NA 

ND NA NA 

42192101 Terrestrial field dissipation (164-1) Bare ground / Silt loam soil 

 Hillsboro, Oregon 

143 days NA NA NA 

42192102 Terrestrial field dissipation (164-1) Bare ground / Sandy loam soil 

Janesville, North Carolina 

64 days NA NA NA 

40003714 Forestry Dissipation (164-3) Aerial application, 

residues measured 

12-40 days (vegetation) 37-44 days (litter) 

NA NA NA 
34 

APPENDIX C- Ecological Effects Data

Studies are with imazapyr acid, unless otherwise noted

71-1 Avian Acute Oral (850.2100)

Bobwhite Quail. MRID 00131633 (Acceptable). In a 14-day oral gavage study, imazapyr acid 

was determined to be practically non-toxic to bobwhite quail with an LD50 of >2,150 mg ae/kg. 

The study is scientifically sound and follows the guideline protocols. 

Mallard Duck. MRID 00131634 (Acceptable). In a 14-day oral gavage study, imazapyr acid was 

determined to be practically non-toxic to mallard ducks with an LD50 of >2,150 mg ae/kg. The 

study is scientifically sound and follows the guideline protocols. 

71-2 Avian Subacute Dietary (850.2200)

Imazapyr acid

Bobwhite Quail. MRID 00131635 (Acceptable). In an 8-day dietary study, imazapyr acid was 

determined to be practically non-toxic to upland game birds (bobwhite quail) with an LC50

>5000 ppm. The study is scientifically sound and generally followed guideline protocols; 

however, there was some unexplainable low weight gains and mortality at the 625 ppm test 

concentration. 

Mallard. MRID 00131636 (Acceptable). In an 8-day dietary study, imazapyr acid was 

determined to be practically non-toxic to mallard ducklings with an LC50 >5000 ppm.. The study 

is scientifically sound and generally followed guideline protocols. 

Imazapyr isopropylamine salt

Bobwhite Quail. MRID 00147115 (Acceptable). In an 8-day dietary study, the isopropylamine 

salt of imazapyr was determined to be practically non-toxic to upland game birds (bobwhite 

quail) with an LC50 >5000 ppm. The study is scientifically sound and generally followed 

guideline protocols. This study was conducted with the formulated product to ensure that 

isopropylamine did not affect the toxicity of the active ingredient. 

71-4 Avian Reproduction (850.2300)

Bobwhite Quail. MRID 45119714a (Acceptable). In a one-generation reproductive toxicity 

study, imazapyr acid produced no evidence of treatment-related adverse effects on adult or 

reproductive parameters with an NOAEC of 1670 ppm. The study is scientifically sound and 

generally followed guideline protocols. 

Mallard. MRID 45119714b (Invalid). In a one-generation reproductive toxicity study, imazapyr 

acid resulted in a significant reduction in the ratio of viable embryos/eggs at the 1,670 ppm 
35 

treatment level. However, the study was determined to be invalid due to bacterial contamination 

and high embryonic mortality in the controls. EFED recommended that another study be 

conducted to determine the reproductive toxicity of imazapyr to waterfowl. 

Bobwhite Quail. MRID 43831401 (Originally Supplemental; Reclassified Core). In a onegeneration reproductive toxicity study, imazapyr acid resulted in reduced hatchlings/live embryo 

at 2000 ppm (LOEC; NOEC = 1000 ppm); however, the study was originally determined to be 

supplemental due to guideline deficiencies (primarily, EECs would be higher than highest dose 

tested and control egg shell cracking was 13%). EFED reevaluated the studies and determined 

that the dosing did reflect the maximum EEC and that the handling and measurement 

deficiencies did not reflect a dose-response relationship; consequently, the study was reclassified 

as core and the NOEC was changed to 2000 ppm. 

Mallard. MRID 43831402 (Originally Supplemental; Reclassified Core). In a one-generation 

reproductive toxicity study, imazapyr acid produced no evidence of treatment-related adverse 

effects on adult or reproductive parameters with an NOAEC of 1890 ppm (measured 

concentration; 2000 ppm nominal concentration). However, the study was originally 

determined to be supplemental due to guideline deficiencies (primarily, EECs would be higher 

than highest dose tested, inaccurate measurement of egg shell thickness, and insufficient pre-egg 

laying period.) EFED reevaluated the studies and determined that the dosing did reflect the 

maximum EEC and that the measurement deficiencies did not reflect a dose-response 

relationship; consequently, the study was reclassified as core and the NOAEC was established at 

2000 ppm. 

72-1 Freshwater Fish Acute (850.1075)

Imazapyr acid

Rainbow Trout. MRID 00131629 (Acceptable). In a 96-hour acute test, imazapyr acid was 

determined to be practically non-toxic to rainbow trout with an LC50 of >100 mg/L. The NOEC 

was determined to be 100 mg/L. The study is scientifically sound and meets guideline protocols. 

Bluegill Sunfish. MRID 00131630 (Acceptable). In a 96-hour acute test, imazapyr acid was 

determined to be practically non-toxic to bluegill sunfish with an LC50 of >100 mg/L. The 

NOEC was determined to be 100 mg/L. The study is scientifically sound and meets guideline 

protocols. 

Channel Catfish. MRID 00131631(Acceptable). In a 96-hour acute test, imazapyr acid was 

determined to be practically non-toxic to channel catfish with an LC50 of >100 mg/L. The 

NOEC was determined to be 100 mg/L. The study is scientifically sound and meets guideline 

protocols. 

 
36 

Imazapyr isopropylamine salt

Rainbow Trout. MRID 45119713 (Acceptable). In a 96-hour flow-through test, imazapyr 

isopropylamine salt was determined to be practically non-toxic to rainbow trout with an LC50 of 

>110 mg ae/L (mean measured concentration; nominal concentration 120 mg ae/L). The NOEC 

was determined to be 110 mg ae/L. The study is scientifically sound and meets guideline 

protocols. 

Bluegill Sunfish. MRID 00147116 (Acceptable). In a 96-hour test, imazapyr isopropylamine salt 

was determined to be practically non-toxic to bluegill sunfish with an LC50 of >818 mg ae/L 

(1000 mg ai/L). The study is scientifically sound and meets guideline protocols. 

72-2 Freshwater Invertebrate Acute (850.1010)

Imazapyr acid

Daphnia. MRID 00131632 (Core). In a 48-hour acute test, imazapyr acid was determined to be 

practically non-toxic to daphnids with an EC50 of >100 mg/L. The study is scientifically sound 

and meets guideline protocols. 

Imazapyr isopropylamine salt

Daphnia. MRID 00147117 (Core). In a 48-hour static test, imazapyr isopropylamine salt was 

determined to be practically non-toxic to daphnids with an EC50 of 614 mg ae/L (750 mg ai/L). 

The study is scientifically sound and meets guideline protocols. 

72-3a Estuarine/Marine Fish Acute (850.1075)

Silverside Minnow. MRID 41315801 (Acceptable). In a 96-hour flow-through test, imazapyr 

acid was determined to be practically non-toxic to silverside minnow with an LC50 of >184 mg 

ai/L (mean measured concentration; nominal concentration 200 mg ai/L). The NOEC was 

determined to be 184 mg ai/L. The study is scientifically sound and meets guideline protocols. 

72-3b Estuarine/Marine Invertebrate Acute (850.1025, 850.1045)

Eastern Oyster. MRID 45119710 (Acceptable). In a 96-hour flow-through test, imazapyr acid 

was determined to be practically non-toxic to the eastern oyster with an EC50 of >132 mg ai/L 

(mean measured concentration; nominal concentration 120 mg ai/L). No mortalities were 

observed in either the treated or control groups. The control shell deposition during the study 

was 2.46 mm. The NOAEC was determined to be 132 mg ai/L, the highest concentration tested. 

No significant adverse effects were observed on shell deposition for any treated group. The 

study is scientifically sound and meets guideline protocols. 

Eastern Oyster. MRID 41315802 (Supplemental). In a 96-hour flow-through test, imazapyr acid 

was determined to be practically non-toxic to the eastern oyster with an EC50 of >173 mg ai/L 

(mean measured concentration; nominal concentration 200 mg ai/L; the highest concentration 

tested). No mortalities were observed in either the treated or control groups. There was a 

statistically significant decrease in mean shell deposition at 173 mg/L when compared to the 
37 

control group (p  0.05). The NOAEC was determined to be 109 mg ai/L. Originally, this study 

was classified as invalid because the control oyster growth (1.35 mm new shell deposition) did 

not meet the guideline requirement of 2 mm (amendment to SEP, dated 9/1990). In addition, the 

flow rate of the test solution was 1.05 L/oyster/hour. The protocols recommended by the SEP 

(APHA, 1981 and EPA, 1976) state that each oyster should receive a minimum of 5 

L/oyster/hour. This study was later upgraded to supplemental. The memorandum stated that 

shell growth in the control group may be used as an indicator of stress for the oysters. Less than 

2 mm shell growth in the control group indicates that the oysters may be undergoing stress. The 

low flow-through rate with no supplemental food added in this study may have contributed to 

stress on the oysters. For this study, it appears that the seawater was trucked in from the ocean to 

Gainesville, Florida. During such time, the food organisms (such as algae) may have been 

inhibited during the transport and storage. The oysters in the study may not have fed well 

because of the combination of the lower amount of available food organisms in the shipped sea 

water, the low flow rate and the lack of supplemental food added; thereby, contributing to 

inadequate shell deposition. However, since there was some dose-response, the study was 

upgraded to supplemental. 

Pink Shrimp. MRID 41315803 (Acceptable). In a 96-hour flow-through test, imazapyr acid 

was determined to be practically non-toxic to pink shrimp with an LC50 of >189 mg ai/L (mean 

measured concentration; nominal concentration 200 mg ai/L). There was one mortality at the 

second highest concentration level (111 mg/L), which does not appear to be related to treatment. 

No other signs of toxicity were observed. Therefore, the NOAEC was determined to be 189 mg 

ai/L, the highest concentration tested. The study is scientifically sound and meets guideline 

protocols. 

72-4a Freshwater Fish Early Life Stage (850.1400)

Fathead Minnow. MRID 45119711 (Acceptable). In an early life-stage flow-through test, 

imazapyr acid produced no treatment-related effects on embryonic survival, time to hatch, alevin 

survival, terminal length, or wet and dry weight. The NOEC was determined to be 118 mg ai/L 

(mean measured concentration; nominal concentration 120 mg ai/L). The study is scientifically 

sound and meets guideline protocols. 

Rainbow Trout. MRID 41315804 (Supplemental). In an early life-stage flow-through test, 

imazapyr acid resulted in significantly reduced percent hatch and an observed reduction on 

survival at 92.4 mg/L (mean measured concentration; nominal concentration 100 mg/L). No 

abnormalities in embryonic or juvenile development were observed. The MATC was >43.1 and 

<92.4 mg/L; thus the geometric mean MATC was 63.1 mg/L. The study did not meet all 

guideline requirements (feeding limited the growth of replicates with higher fish densities). 

72-4b Freshwater Invertebrate Life Cycle (850.1300)

Daphnia. MRID 41315805 (Acceptable). In a life cycle flow-through test, imazapyr acid 

produced no treatment-related effects on survival, growth and reproduction of first generation 
38 

daphnids. No physical or behavioral abnormalities were observed. The MATC and NOEL were 

determined to be 97.1 mg/L. The study is scientifically sound and meets guideline protocols. 

72-5 Freshwater Fish Life Cycle (850.1500)

Fathead Minnow. MRID 45119712 (Supplemental). In a full life cycle flow-through test, 

imazapyr acid produced no treatment-related effects on growth, embryo survival, time to hatch, 

or larval and juvenile survival of the F0 and F1 generations. No treatment-related effects were 

observed on percent spawning frequency, mean number of eggs produced per female or mean 

percent fertilization success. The NOEC was reported at the nominal concentration of 120 mg 

ai/L (mean measured concentration 118 mg ai/L). The study is scientifically valid but did not 

meet all guideline requirements (F1 generation was maintained for 4 weeks instead of 8 weeks). 

81-1 Acute Mammalian Oral 

Imazapyr acid

Rat. MRID 132030 (Acceptable). In an acute oral study, imazapyr acid was determined to have 

a low toxicity (Toxicity Category III) to rats with an LD50 >5000 mg/kg. The study is 

scientifically sound and meets guideline protocols. 

Imazapyr isopropylamine salt

Rat. MRID 00147049 (Acceptable). In an acute oral study, imazapyr isopropylamine salt was 

determined to exhibit no toxicity (Toxicity Category IV) to rats with an LD50 of >5000 mg/kg 

(>4090 mg ae/kg). The study is scientifically sound and meets guideline protocols. 

Rat. MRID 44735301 (Acceptable). In an acute oral study, imazapyr isopropylamine salt was 

determined to exhibit no toxicity (Toxicity Category IV) to rats with an LD50 of >5000 mg/kg 

(>4090 mg ae/kg). The study is scientifically sound and meets guideline protocols. 

83-3 Mammalian Developmental

Rat. MRID 00131611 (Acceptable). In a 2-generation developmental study, imazapyr acid 

produced maternal toxicity in Sprague Dawley rats at 1000 mg ai/kg/day (LOAEL), based on 

salivation in the gravid dams between gestation days 8-15. The findings were determined to be 

treatment-related. The NOAEL was 300 mg /kg bw/day. No treatment-related effects were 

reported for developmental parameters. The study is scientifically sound and meets guideline 

protocols. 

Rabbit. MRID 00131613 (Acceptable). In a 2-generation teratology study, imazapyr acid 

produced no treatment-related effects for maternal or developmental parameters; consequently, 

the NOAEL for both endpoints was 400 mg/kg bw/day in New Zealand white rabbits. The 

study is scientifically sound and meets guideline protocols. 
39 

83-4 Mammalian Reproduction

Rat. MRID 41039505 (Acceptable). In a 2-generation reproduction study, imazapyr acid 

produced no treatment-related effects for maternal or developmental parameters; consequently, 

the parental systemic, reproductive, and offspring NOAEL was 738 mg/kg bw/day in males and 

933.3 mg/kg bw/day in females. The study is scientifically sound and meets guideline protocols. 

122-2 Aquatic Plant Nonvascular (850.5400)

Imazapyr acid

Green algae. MRID 40811802 (Acceptable). In a Tier II toxicity test with Selenastrum 

capricornutum, the 7 day EC50 for cell density was 71 mg ai/L (NOEC = 50.9 mg ai/L). The 

study is scientifically sound and meets the guideline protocols. 

Blue-green algae. MRID 40811802 (Acceptable). In a Tier II toxicity test with Anabaena flosaquae, the7-day EC50 for cell density was 12.2 mg ai/L (NOEC = 9.6 mg ai/L). The study is 

scientifically sound and meets the guideline protocols. 

Marine diatom. MRID 40811802 (Acceptable). In a Tier II toxicity test with Skeletonema 

costatum, the 7-day EC50 for cell density was 92 mg ai/L (NOEC = 15.9 mg ai/L). The study is 

scientifically sound and meets the guideline protocols. 

Diatom. MRID 40811802 (Acceptable). In a Tier II toxicity test with Navicula pelliculosa, the 7-

day EC50 for cell density was >41 mg ai/L (NOEC = 41 mg ai/L). The study is scientifically 

sound and meets the guideline protocols. 

Imazapyr isopropylamine salt

Green algae. MRID 43889102 (Acceptable). In a Tier II toxicity test with green algae, the 7-day 

EC50, based on slight changes in cell shape was 11.5 mg ae/L (NOEC = 7.16 mg ae/L). The 

study is scientifically sound and meets the guideline protocols. 

123-1(a) Seedling Emergence - Tier II (850.4225)

Imazapyr Acid

Monocots (4 species) and Dicots (4 species). MRID 40811801 (Supplemental). In a Tier II 

seedling emergence study, the most sensitive monocot tested was wheat (EC25 0.0046 lb ae/acre, 

EC05 0.00099 lb ae/acre; shoot weight). The most sensitive dicot tested was sugarbeet (EC25

0.0024 lb ae/acre, EC05 0.00017 lb ae/acre; shoot weight). Due to deficiencies in the study, the 

guideline requirements are only partially fulfilled; acceptable data endpoints were used in the 

risk assessment. 

 
40 

Nontarget Terrestrial Plant Seedling Emergence Toxicity (Tier II)

Species % ai EC25 (lbs ai/A) NOEC / 

[EC05] 

Endpoint Affected MRID No. Study Classification 

22.6% 408118-01 

Monocot- Corn 0.025 0.0156 height invalid2

Monocot- Oat 0.054 0.0156 “ supplemental3

Monocot- Onion 0.034 [0.01]A weight4

 supplemental3

Monocot- Wheat 0.0046 [0.00099]A “ supplemental3

Dicot- Sunflower 0.0027 [0.000021]A height invalid2

Dicot- Soybean 0.012 0.0078 “ invalid2

Dicot- Pea 0.093 0.0624 weight4

 invalid2

Dicot-Cucumber 0.0043 [0.000005]A “ invalid2

Dicot- Sugarbeet 0.0024 [0.00017]A “ supplemental3

Dicot- Tomato 0.008 0.0003 “ supplemental3

1

Determination of the most sensitive species is based on EC25 values; results are based on the non-linear 

regression analysis. 

2

 Large seedlings were subjected to overcrowding, 10 seeds were planted in a 4-in dixie cup. 

3

 Small seedlings could be subjected to overcrowding, 10 seeds were planted in a 4-in dixie cup. 

4

 Fresh weight was recorded instead of dry weight. 

A

 The NOEC value is above the EC25, equal to the EC25, or below the lowest concentration, an EC05 

value is used instead. . 

123-1(b) Vegetative Vigor - Tier II (850.4250)

Imazapyr acid

Monocots (4 species) and Dicots (4 species). MRID 40811801 (Supplemental). In a Tier II 

vegetative vigor study, the most sensitive monocot tested was wheat (EC25 0.012 lb ae/acre, 

NOEC 0.0039 lb ae/acre; shoot weight). The most sensitive dicot tested was cucumber (EC25

0.0009 lb ae/acre, EC05 0.000064 lb ae/acre; shoot height). Due to deficiencies in the study, the 

guideline requirements are partially fulfilled; acceptable data endpoints were used in the risk 

assessment. 
41 

Species 

% a.i. EC25 

(lbs 

ae/A) 

NOEC [EC05] 

(lbs ae/A) 

Endpoint 

affected 

MRID No. Study 

classification 

22.6% 408118-01 

Monocot-Corn >0.0156A 0.0078 weight2

 supplemental3

Monocot-Oats 0.013 0.0039 height supplemental3

Monocot-Onion -- -- n/aC invalid 

Monocot-Wheat 0.012 0.0039 weight2

 supplemental3

Dicot-Soybean -- -- n/aC invalid 

Dicot-Pea -- -- n/aC invalid 

Dicot-Sugarbeet 0.00097 [0.00039]B weight2

 supplemental3

Dicot-Sunflower 0.0054 0.0039 weight2

 supplemental3

Dicot-Cucumber 0.0009 [0.000064]B height supplemental3

Dicot-Tomato >0.0156A 0.00097 weight2

 supplemental3

1

Determination of the most sensitive species is based on EC25 values. 

2 Fresh weight was recorded instead of dry weight. 

3

 the toxicity values could be underestimated since the study was tested with older plants (28D) at a less 

sensitive stage of growth (timing of application). 

A The EC25 value is above the highest concentration tested. 

B

 The NOEC value is above the EC25, equal to the EC25, or below the lowest concentration, an EC05 

value is used instead. 

C

 No data

Imazapyr isopropylamine salt

Monocots (3 species) and Dicots (2 species). MRID 43889101 (Core). In a Tier II vegetative 

vigor study, chlorosis, stunting, and plant death were observed. The most sensitive monocot 

tested was onion (EC25 0.010 lb ae/acre, NOEC 0.004 lb ae/acre; shoot weight). The most 

sensitive dicot tested was sugar beet (EC25 0.0016 lb ae/acre, NOEC 0.0008 lb ae/acre; shoot 

weight). The study is scientifically sound and meets guideline protocols. 

Monocots (4 species) and Dicots (4 species). MRID 40003711 (Supplemental). This study was 

a modified Tier II vegetative vigor study that did not meet guideline requirements. Only 
42 

descriptive summary data was presented; consequently effect levels were not determined. 

Observed effects included chlorosis, stunting, leaf tip burning, necrosis, and plant death. 

123-2 Aquatic Plant Vascular (850.4400)

Imazapyr acid

Duckweed. MRID 40811802 (Acceptable). In a 14-day toxicity test with duckweed, the EC50 for 

frond production was 0.024 mg ai/L and the NOEC was 0.01 mg ai/L. Imazapyr is considered 

highly toxic and expected to exert a detrimental effect on vascular aquatic plants. The study is 

scientifically sound and meets guideline protocols. 

Imazapyr isopropylamine salt

Duckweed. MRID 43889102 (Acceptable). In a 14-day toxicity test with duckweed, the EC50 for 

frond production was 0.018 mg ai/L and the NOEC was 0.011 mg ai/L. The study is 

scientifically sound and meets guideline protocols.
APPENDIX D 

The Screening Imbibition Program (SIP v.1.0, Released June 15, 2010) was used to calculate an 

upper bound estimate of exposure using imazapyr’s solubility (acid: 11,100 mg/L), the most 

sensitive acute and chronic avian toxicity endpoints (bobwhite quail LD50>2,150 mg ae/kg-bw, 

MRID 00131633; and bobwhite quail NOAEC of 1670 mg ae/kg-diet, MRID 45119714 as well 

as mallard duck NOAEC of 1890 mg ae/kg-diet, MRID 43831402) and the most sensitive acute 

and chronic mammalian toxicity endpoints (rat LD50>5,000 mg ae/kg-bw, MRID 00132030; and, 

a rat NOAEL of 300 mg/kg-bw/day, MRID 00131611). Drinking water exposure alone was 

determined to be a potential pathway of concern for avian and mammalian species on an acute 

and chronic basis. However, given that most studies indicated no effects (with the exception of 

the rat developmental study, MRID 00131611) it is possible that the result is largely due in part 

by the high solubility limit of imazapyr. 

This pathway will be explored further with the development of SIP v.2.0 in the Ecological Risk 

Assessment for imazapyr. See table below for a sample of the output generated by SIP v.1.0. 

Detailed information about the SIP v.1.0, as well as the tool, can be found on the EPA’s website 

at http://www.epa.gov/pesticides/science/models_pg.htm#terrestrial. 

 

Table 1. Inputs 

Parameter Value 

Chemical name Imazapyr (acid) 

Solubility (in water at 25oC; mg/L) 11100 

 

Mammalian LD50 (mg/kg-bw) 5000 

Mammalian test species laboratory rat 

Body weight (g) of "other" mammalian 

species 

 

Mammalian NOAEL (mg/kg-bw) 300 

Mammalian test species laboratory rat 

Body weight (g) of "other" mammalian 

species 

 

Avian LD50 (mg/kg-bw) 2150 

Avian test species northern bobwhite quail 

Body weight (g) of "other" avian species 

Mineau scaling factor 1.15 

 

Mallard NOAEC (mg/kg-diet) 1890 

Bobwhite quail NOAEC (mg/kg-diet) 1670 

NOAEC (mg/kg-diet) for other bird species 

Body weight (g) of other avian species 

NOAEC (mg/kg-diet) for 2nd other bird 

species 

Body weight (g) of 2nd other avian species 
44 

 

 

 

 

 

Table 2. Mammalian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 1909.2000 1909.2000 

Adjusted toxicity value (mg/kg-bw) 3845.8028 230.7482 

Ratio of exposure to toxicity 0.4964 8.2740 

Conclusion* 

Exposure through 

drinking water alone is a 

potential concern for 

mammals 

Exposure through 

drinking water alone is a 

potential concern for 

mammals 

 

Table 3. Avian Results 

Parameter Acute Chronic 

Upper bound exposure (mg/kg-bw) 8991.0000 8991.0000 

Adjusted toxicity value (mg/kg-bw) 1548.9235 93.7677 

Ratio of exposure to acute toxicity 5.8047 95.8859 

Conclusion* 

Exposure through 

drinking water alone is a 

potential concern for 

birds 

Exposure through 

drinking water alone is a 

potential concern for 

birds 

 

*Conclusion is for drinking water exposure alone. This does not combine all routes of exposure. 

Therefore, when aggregated with other routes (i.e., diet, inhalation, dermal), pesticide exposure through 

drinking water may contribute to a total exposure that has potential for effects to non-target animals. 
45 

APPENDIX E 

The Screening Tool for Inhalation Risk (STIR v.1.0, November 19, 2010) was used to calculate 

an upper bound estimate of exposure using imazapyr acid’s vapor pressure (<10-7 mm Hg) and 

molecular weight (293.16 g/mol) for vapor phase exposure as well as the maximum application 

rate (1.5 lb a.i./A) and method of application for spray drift (i.e., ground and aerial spray). STIR 

incorporates results from several toxicity studies including acute oral and inhalation rat toxicity 

endpoints obtained from the “six-pack” of core studies, which are a series of six guideline studies 

that are submitted to the Registration Division of the Office of Pesticide Programs for technical 

and formulated products of a pesticide. The technical grade acute inhalation study (Guideline 

870.1300) data was obtained from an HED risk assessment (US EPA 2010): LC50> 1.3 mg/L 

(MRID 00252004); the typical duration for these studies is 4 hours, which was assumed for this 

study as well. The most sensitive acute oral avian toxicity endpoint (bobwhite quail LD50>2,150 

mg ae/kg-bw, MRID 00131633) and most sensitive acute mammalian endpoint (rat LD50>5,000 

mg ae/kg-bw, MRID 00132030) were obtained from ecological toxicity studies submitted to 

EFED. Based on the results of the STIR model, inhalation exposure alone was not determined to 

be a potential pathway of concern for avian and mammalian species on an acute basis. 

Inhalation exposure via spray drift and/or vapor-phase of the pesticide alone does not appear to 

be of concern. The analysis of the inhalation route in STIR does not consider that aggregation 

with other exposure pathways such as dietary, dermal, or drinking water may contribute to a total 

exposure that has a potential for effects to non-target animals. However, the Agency does 

consider the relative importance of other routes of exposure in situations where data indicate that 

pesticide exposures through other routes may be potentially significant contributors to wildlife 

risk (USEPA, 2004). Detailed information about STIR v.1.0, as well as the tool, can be found on 

the EPA’s website at: http://www.epa.gov/pesticides/science/models_pg.htm#terrestrial. 

References; 

USEPA. 2004. Overview of the Ecological Risk Assessment Process in the Office of Pesticide 

Programs. U.S. Environmental Protection Agency, Office of Prevention, Pesticides and 

Toxic Substances, Office of Pesticide Programs, Washington DC. January 23, 2004. 

USEPA. 2010. Imazapyr: Revised Occupational and Residential Exposure/Risk Assessment 

Under 24(c) for the Proposed Use of Broadcast Application on Cogongrass in Areas that 

may be Grazed or Cut for Hay. U.S Environmental Protection Agency, Office of 

Chemical Safety and Pollution Prevention, Office of Pesticide Programs, Washington 

DC. July 27, 2010. 
46 

Sample input/output:

Input 

Application and Chemical Information 

Enter Chemical Name Imazapyr 

Enter Chemical Use various 

Is the Application a Spray? (enter y or n) y 

If Spray What Type (enter ground or air) air 

Enter Chemical Molecular Weight (g/mole) 261.28 

Enter Chemical Vapor Pressure (mmHg) 1.00E-07 

Enter Application Rate (lb a.i./acre) 1.5 

 

Toxicity Properties 

Bird 

Enter Lowest Bird Oral LD50 (mg/kg bw) 2150 

Enter Mineau Scaling Factor 1.15 

Enter Tested Bird Weight (kg) 0.178 

Mammal 

Enter Lowest Rat Oral LD50 (mg/kg bw) 5000 

Enter Lowest Rat Inhalation LC50 (mg/L) 1.3 

Duration of Rat Inhalation Study (hrs) 4 

Enter Rat Weight (kg) 0.35 

 

Output 

Results Avian (0.020 kg ) 

Maximum Vapor Concentration in Air at Saturation 

(mg/m3) 1.58E-03 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 1.98E-04 

Adjusted Inhalation LD50 

3.12E+0

0 

Ratio of Vapor Dose to Adjusted Inhalation LD50 6.36E-05

Exposure not Likely 

Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 1.44E-01 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 4.62E-02

Exposure not Likely 

Significant 

 

Results Mammalian (0.015 kg ) 

Maximum Vapor Concentration in Air at Saturation 

(mg/m3) 1.58E-03 

Maximum 1-hour Vapor Inhalation Dose (mg/kg) 2.49E-04 

Adjusted Inhalation LD50 

7.74E+0

1 

Ratio of Vapor Dose to Adjusted Inhalation LD50 3.22E-06

Exposure not Likely 

Significant 

Maximum Post-treatment Spray Inhalation Dose (mg/kg) 1.81E-01 

Ratio of Droplet Inhalation Dose to Adjusted Inhalation 

LD50 2.34E-03

Exposure not Likely 

Significant